As noted above, sometimes you can find a model made by someone else on Thingiverse, Printables, STLfinder, etc.
Also, McMaster-Carr often provides 3D models for many of their products, so you can import those.
But often you still will need to modify what you find online, which generally means CAD.
(You can do numerical scaling in slicing programs like Cura, without needing CAD.)
I personally use FreeCAD, which is great, but it's not the place to start if you're new to CAD.
You also can get a free 1 year license for Fusion 360, but it's also not for beginners and it has a fairly hefty annual license fee afterwards.
Also noted above, TinkerCAD is the one usually suggested for those new to CAD.
There are many online blogs to help you get started with CAD.
All3DP.com can be a good place to start - articles are very easy to digest (albeit light on details). But it can point you in the right direction for many things - including what software programs do certain tasks, which are best for your needs (including beginners), material choices, plus finding useful blogs for beginner CAD design. It's been many years since I started 3D printing, so I've forgotten the names of most of the various blogs - but some I remember are CNC Kitchen, Toglefritz's Lair, 3D Printerly, etc.
Good luck!
Note added: FreeCAD is freeware
https://wiki.freecad.org/Download