Instead of looking at big dealerships (where a lot of them contract out their detail work for small dollars to people of questionable immigration status...not a slam, but reality), you might concentrate your efforts at small, high end pre-owned car dealers. I ended up at a pre-owned BMW dealer that sells about 100 cars per year and couldn't be happier.
I'd look for BMW, Porsche, Mercedes used car dealers. Most of these folks buy cars coming off lease at auctions. They usually are 3 years old, have 35-50/k miles and are in decent shape. These dealers are typically looking for extra sharp detailing because it gives them an edge over the big, franchised stores. Most will want to see some examples of your work, so be prepared for that.
Toto