Over the years it is getting easier and better to find a file than to get a pdf and have to try and make a model from it for just a representation of a part that you want to use.  Some of the manufacturers especially on the electrical side really want you to know who's model it is that you are looking at when they let you download one.  

Air compressor vessels, spvgstocktanks.com

Electrical components if you can't find them on the mfg web try searching here  3dfindit.com

Then my favorite place to find just about anything from bolts to hose reels ect.   
mcmaster.com

The only model that is difficult to find in 3d has been a transformer, that would be used inside of a electrical enclosure.  I used to go to 3D content central but some companies just uploaded a catalog of information with no 3D files and searching for the part that you wanted you could end up spending 30 min scrolling thru pages of results.  
My other new one has been to upload a pdf to AI and see if it could find me a model.  It worked ok on some items but not great on others.