I just learned this:
A pal took his make, model, year and paint code to his NAPA store and they made him a can of spray paint for a replacement mirror. The match was perfect.
He suggested 2 cans for a fender.
As always, prep is everything.

Also, there are places online. Autotouchup.com offers cans, spray cans, broad tipped and finely tipped pens. I had to wait two weeks for a fine tipped pen in an "unstocked color". You may prefer to avoid shipping time lag by going to NAPA.

As mentioned, there are auto body supply shops which provide the same service.