Over the years, it has gotten easier and easier to find a 3D model instead of getting a PDF and having to try to create a model from it just to have a representation of the part you want to use.

Some manufacturers, especially on the electrical side, really want you to know whose model you are looking at when they let you download one. 😄

Here are some of the places I have found useful:

Air Compressor Vessels
SPVG Stock Tanks
spvgstocktanks.com

Electrical Components
If you can't find what you are looking for on the manufacturer's website, try:
3dfindit.com

rittal.com

nvent.com

https://support.industry.siemens.com

https://telemecaniquesensors.com/

General Hardware, Components, etc.
My favorite place to find just about anything, from bolts to hose reels and everything in between:
mcmaster.com

Another good source:
TraceParts
traceparts.com/en
(Steven Dod)

The one type of model that has been difficult for me to find in 3D is a transformer that would be used inside an electrical enclosure.

I used to use 3D ContentCentral, but some companies would upload a catalog of information without actually providing 3D files. You could end up spending 30 minutes scrolling through pages of results trying to find the part you wanted.

One of my newer methods has been to upload a PDF to AI and see if it can find me a model. It has worked pretty well on some items, but not so great on others.

If anyone has other good sources for finding 3D CAD models, especially manufacturers or sites that provide free models, I'd be interested in hearing about them.