Where can I buy professional quality automotive paint for DIY use?

Just search online for “Automotive refinishing paint suppliers” in your area. 

Just be aware that you still have to thin the paint correctly to suit your paint sprayer, and that twin-pack paints are highly toxic and can only be safely applied in a full protective suit with an air supply and in a closed paint booth with a filtered exhaust. That sort of equipment is not in DIY territory. 

There are non-toxic alternatives, and some paint suppliers will still supply the old cellulose paints where legally permitted. Many jurisdictions ban their usage due to their very high VOC content.

Do a search for automotive paint supplies in your area. We have a NAPA store that sells automotive paint. Not every NAPA store sells paint.

Google "Sikkens", "Glasurit", "Spies Hecker", "RM".  It will lead you to the store.

NAPA