I used to get it by the case (6pk) from a Wurth vendor that sold to all the local auto dealerships. Wurth sells all sorts of neat stuff, from door panel clips to really neat threaded rivet inserts. It's a very big company with really great products, and not so great prices.
The only place I've seen the paint (outside of my local wurth vendor) is on Autogeek, and they wanted $15 a can plus shipping. Not cheap. I used to pay around 6 or 8 a can I think. I used tons of the bumper black (satin black trim paint), and can vouch for it all day long. I tried some auto parts store "bumper black" a couple times, and I found that it took twice as much paint or more, and it most often had problems with crazing, fish eyes, etc. The Wurth stuff is just the right shade of low gloss black, and if you put it on in very thin coats and build it up, starting with a tack coat, it really does well about not fish eyeing. And the end result is a nice even correctly colored finish.
They have a dark gray as well, that matches things like plastic grilles and such. They also have wheel paints, and I hear rave reviews about them at autogeek, but haven't tried them myself. In my humble opinion, wheel paint should be powdercoat like it is from the factory. But if you're just touching up some wheels and it isn't a show car, their wheel paint is probably a very good choice.
I hope you can find the Wurth paint cheaper, maybe Ebay? But if not, buy it anyway. It's that good. Scuff the surface up, clean it, dry it, and do 1 light tack coat. After that, 1 - 2 more light coats, followed up by a final medium coat. No heavy coats at all. The results are great, and you'll be surprised how long a can will last.