raymond m
I don't know if you can buy them. The best thing to do would be paint a primed canvas with black gesso. Be sure to sand the surface with fine sand paper before painting the gesso. You can buy black gesso.

tlw-artist
Black gesso. You can also buy grey gesso and gold, rust red and a few other colors.

Beyond Mordant
The first two Answerers have it covered...or you can spray paint a primed canvas with matt black acrylic and save a lot of effort. Oil paint works fine on dry acrylic.

hushcolours
Hi, Besides the mentioned options, I've seen black canvases from Pebeo, here in Lisbon. Maybe you can look for Pebeo in your country. But I guess it's cheaper to tint a primer or use a black one. Besides this way you can prime it to your own taste. Kind regards, José