The Thingiverse

An alternative to making your own 3D file using CAD is to download a ready made version from an online repository

3D files come in many forms but one of the most common the .stl file.

STL stand for Standard Tessellation Language

Here are some good online resources (not all models are free):

Thingiverse - Digital Designs for Physical ObjectsThingiverse - Digital Designs for Physical Objects

Cults・Download free 3D printer models・STL, OBJ, 3MF, CADCults 3D

YouMagine - A Community for RC Creators & Makerswww.youmagine.com

Probably the most similar to Thingiverse

Discover STL files for 3D printing ideas and high-quality 3D printer models. | MyMiniFactorywww.myminifactory.com

Pinshape - 3D Marketplace for Designerspinshape.com

https://grabcad.com/grabcad.com

Aimed more at professional engineers

You have to pay for the ones in CGTrader

CGTrader - 3D Model StoreCGTrader