How to Find 3D CAD Files and Content for 3D Printing

I want to print something, but I don't want to (or can't) design it in CAD? 

Where can I find 3D CAD files that I can download and use on my 3D printer?

There are many repositories of 3D content online, varying from 3D printable objects, to engineering CAD models, to rendering and 3D digital design content. Not all parts are able to be printed on a 3D printer (due to the complexity and thin lines usually), but for the majority of things you may need, there is a CAD model available!

Below is a list of sites that I have found, and a little blurb about what they can supply. This list is not in any particular order other that the random order that I originally found them in when I first compiled this list a while back, and isn't a full list of everything out there - just the sites I found most useful and usable so far.

Here is a list of major 3D Content Sites.

Name: Thingiverse
Site: http://www.thingiverse.com
Review: This site is one of the few that is dedicated to providing access to CAD files that are designed for 3D printing. There is also plenty of reviews on how to print the objects, and pictures of other people's prints. Excellent website, and the first place you should look for a 3D part! No login required.
STL Availability: YES
Cost: FREE

Name: Pinshape
Site: https://pinshape.com/
Review: Here you will find a great amount of good quality files that have been optimised for 3D printing. There are some excellent files here, with good site organisation. Additionally, there is the option for paid parts on this site, so you can monetise your creations if you so wish!
STL Availability: Yes
Cost: Mixed (Some free, some paid)

Name: STL Finder
Site: http://www.stlfinder.com/
Review: This site has a decent mix of stl files available, though it could benefit from some grouping and categories. As the name implies, there are stl's available, and they are free, but thee are also paid options. This is a linking site, so you will be re-directed to the site that the file is hosted on to download it (such as thingiverse etc.).
STL Availability: Yes (re-directed)
Cost: Mixed (Re-directed)

Name: 3D Content Central
Site: http://www.3dcontentcentral.com/
Review: This site is run by Dassault Systemes as a supply site for Solidworks parts, so many parts on here are engineering based, and are in .SLDPRT format (Solidworks part format). As such, if you need something technical, you can come here, but make sure you have Solidworks so you can convert the part to .stl. Site requires a signup to download. I used this site for 3D objects when I use dto do CAD work in Solidworks.
STL Availability: Unsure
Cost: FREE

Name: Autodesk 123D
Site: http://www.123dapp.com/
Review: This site has quite a few models that have been created using Autodesk Software, so these parts are generally quite detailed. You will find here plenty of parts that are not made for 3D prints, many have been created for use in image renders and other digital 3D world uses. Great selection though. Requires signup for download.
STL Availability: Files are provided in multiple formats, and usually include an .stl file.
Cost: Free

Name: 3D Part Source
Site: https://www.3dpartsource.com/
Review: The majority of parts from here seem to come from traceparts online They are both the same company?), which requires a signup. Parts are very industrial and engineering based. OK selection for engineering components.
STL Availability: No
Cost: FREE

Name: Create This
Site: http://www.createthis.com/
Review: Doesn't have many designs yet, though designs that are up are fairly good quality. Selection is a bit low, and paid parts don't really look that amazing. No login required for free parts.
STL Availability: Yes
Cost: MIXED (Some free, some paid)

Name: Cuboyo
Site: http://www.cuboyo.com/
Review: Has some website issues so I couldn't get to some sections. Items are priced pretty low if they do have a price on them, which is nice, but there are still plenty some free objects. Unfortunately there just isn't that many objects yet! Login required to download parts.
STL Availability: Yes
Cost: Mixed (Some free, some paid)

Name: GrabCAD
Site: https://grabcad.com
Review: Has some technical and engineering designs, but also has general stuff too. Quality is generally pretty good, and selection is quite good. Limited .stl files, though files are in various other formats for other programs, so can be converted (if you have the right programs). Requires a login to download.
STL Availability: Very few (other formats available)
Cost: FREE

Name: SketchFab
Site: https://sketchfab.com/
Review: Has some very nice looking objects with great quality, though this high quality would suit them more for renders or digital content rather than 3D printed items. Still, good selection. Requires a login to download, and patrs don't always come in stl so they may have to be converted.
STL Availability: Some
Cost: Mixed (Some free, some paid)

Name: 3D Warehouse - Sketchup
Site: https://3dwarehouse.sketchup.com/
Review: Seems very architectural based, so parts are not optimised for 3D printing. No login required for downloads, though the parts are in sketchup or Collada file formats, so would need to be converted before use. Objects have OK quality, and there is decent selection of buildings and other architectural models.
STL Availability: No
Cost: FREE

Name: Turbosquid
Site: http://www.turbosquid.com/
Review: Has lots of designs, but seem to mostly be for digital use (such as in rendering and videos / games etc.). Models are mostly paid, and requires a login to download. Very nice parts though! Great detail. Part files are not in .stl so conversion is required too.
STL Availability: No
Cost: PAID

Name: Yeggi
Site: http://www.yeggi.com/
Review: Searches OTHER 3D content sites, so has a good selection of multiple genre parts. Since this site links to other sites, downloads are based on the other sites' dowload requirements, and clicking on eachpart will bring you to the site where you can download them from.
STL Availability: Depends on site linked (Mostly yes)
Cost: Based on the site it links to (Mostly free)

Name: 3DaGoGo
Site: https://3dagogo.com/
Review: The designs are given with a breakdown of how long it might take, how to print it nicely, and the 'difficulty', which is great information to have. Some parts free, some not, but has a good selection of good quality parts. Signup is required to download parts, but this is quick and easy.
STL Availability: Yes
Cost: Mixed (Some free, some paid)

Name: Youmagine
Site: https://www.youmagine.com/
Review: Good site similar to thingiverse - parts are generally designed for 3Dprinting, there is a good selection of parts, and they are good quality. Downloads are free and no login is required. Downloads are also already in .stl format which is great.
STL Availability: Yes
Cost: FREE

Name: The Replica Prop Forum
Site: http://www.therpf.com/
Review: This site is project and forum based - so if you are able to find a build that someone has done where they allow and provide the .stl files for public use, you can go ahead and use them (pending permissions from the owner). The files on here can very significantly, since they are project based. Great site though, and has great guides for helping to build solid quality props.
STL Availability: Some
Cost: FREE

Do you know of any more sites that supply 3D objects? 

Let us know in the comments section below, or contact us via the contact form and we will add it to the list! 

I want this list to keep on growing into the best list available on the web, so suggestions are very welcome.

For more info on 3D Printing and CAD, check out the below books available on Kobo.