The Autoparts store is a good way to go.
We(mygirlfriend, a pinstriper, and me) get our stuff from several shops,
we get primer(spray) from a sort of dollarstore, its cheap but works with almost any other spray we put over it.
For some collors we go to two local d.i.y. stores, they sell various quality spraypaint, and some special(flake)paints.
then there's a shop that we go to that is purely a specialized paint shop that selss al kinds of paints from home to car and boat. also has some flake paints.
recently we got a tip from afriend that is heavy into graffity stuff, his paintshop sells various flakes, in spraycans.
if you know anyone that is into graffity and is a good artist, they would/should know a lot about paintcollors and thier chemistry, you could try and ask those guy's.
Most flakespray has a smaller flake then what you see on custom cars though, because the flake has to be small enough to fit to the spraycan nozzle, you can get different nozzles but their use requires some experience/practise.
hope this ads a little to complete your answer.