Do you know if the boxes have been anodized? If so, you will have to strip them before painting. I deal in anodized aluminum product all the time and it can be a bear. You will need to prime them before painting, also. Spray cans are tough, as mentioned.
I have had good luck with powder coaters here in my area. You will have to take everything, and I mean everything out of them before they can paint them though. They paint with black all the time, so should not have to charge all that much to run them along with another order. Try some race or speed shops if you can't find anything in the yellow pages under protective coatings. They have different sheens available, so you can get gloss, flat or satin. It may have a little orange peel in it, depending on the powder and the painter, so ask to see a sample that they have painted to see if you like the finish or not.