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.
When you live in a country where upload speeds are abysmall even when download speeds are ok, you learn to heavily throttle torrent uploads lest you ruin the internet for your entire household.
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.
I actually giggled at this.