To answer your question, almost any body shop can powder coat your wheels. The thing is, not all shops have the machinery to remove your tire. So if you took the wheels alone to any shop it should not be a problem. Powdercoating is a pretty popular method of installing a tough paint job. After they are done you'll then have to take them to have the tires mounted. I've never heard of anybody doing it with the tires still on and if they did it would might come out like shit. I would say the cost would be between 25-75 a piece depending on prep and the shop. If you do go black, go with a flat black and then the clear coat. Gloss black and clear will make too glossy. Flat and clear will still be glossy but with a low tone to it. Good luck and let us know how it turns out.