The best general advice I can give is the best way to find a reputable shop, and that is "always" by word of mouth. If you're like me you attend alot of shows and cruise ins. You will typically see alot of nice work. Ask where it was done and how thorough it was done, not how much. There are alot of great painters in my area of town with great reputations. The best ones are the most expensive and rightfully so. It is hard work and is a specialty. A good shop stays busy with Classic cars, musclecars, hotrods, ect because enthusiast will seek out the good shops. If the shop is full of run of the mill daily drivers it is typically a discount 'knock it out" and "move to the next one" shop. I would avoid those, "even if they are cheaper".
Reputation is more important than how much $. You will pay for a good thorough paint job.