look in the phone book for Sherwin Williams Auto Paint store.
SW not only makes paints for your home but also for your car. and they have stores that just supply the auto body business. granted they might not be as many as they are for home paints but they are out there.
if you have no luck, try going to you regular SW paint store and ask if they can get the supplies you need from their nearest SW Auto Paint store. they should be able to help you out.
or try another brand paint store, like my local benny moore supplier has those paper mesh strainer that the auto body guys use. but the measuring cups they have aren't the ones you need.
if your wondering i run a commerical painting company.
or try an online supplier, have you checked eastwood?