Depends on where you are. There are paint places everywhere, you just need one that does automotive lacquer. As Dedman says the local panel supply place is a good start.
My reservation with SCA is that the people on the floor, in my experience, don't really have the best handle on the products they are selling. It might vary from outlet to outlet, but here they are useless. Following a paint code recipe is one thing, experience with mixing to the point where you can colour match is a different ball game. PMS is also more for ink printing, so the mix is defined by CMYK. Paint mixing works differently as there are different components to the paint (pigment/suspension medium/solvents/hardener) whereas ink is generally solvent and pigment.
Your best bet is to take a chip sample (or your pms book) somewhere to get the colour you want matched.
Otherwise, the SCA patch up rattle can inventory is pretty big. You might find something close enough in there.
Out of interest what PMS colour do you want? Can you give us the code?