My apologies, I didn't mean to be condescending, just meant this as a light jab!
Due to the original post, I assumed you were on linux, which is pushing the use of user accounts pretty aggressively.
If I don't design them myself, I get them from Printables.com.Students come to me with models from Makerworld, and those are, more often than not, terribly optimized for 3D printing.
Also, if you're making raspberry pi enclosures as I think I read in the comments, there is this library that has pre-modeled rpis to use as a reference: https://github.com/nophead/NopSCADlib
It's honestly pretty powerful, you can even import libraries from other users. A good example I've found is the board game inserts that user "js500" is uploading on Printables. He has his own library with things like rounded cubes, finger holes and whatnot, then uses that to create inserts for different games. Here's an example: https://www.printables.com/model/1192543-white-castle-with-matcha-insert-organizer-also-fit
(Make sure to also download the library file included)
I wouldn't recommend Blender for anything that needs to have accurate dimensions, as it is a pain to get things right, even with the "CAD" addon.
FreeCAD seems great, but the UI and UX are horrendous. It's a steep learning curve.
If you have any coding experience, I recommend giving OpenSCAD a look, like mentioned in another comment. I believe most of the older Prusa printer parts were designed on it.
Just a clarification for anyone reading this comment and not the article, these are solitary bees, no hives.