I would presume you would never want to buy a single ball, but only a whole set. Slight manufacturing deviance of balls produced by different machines or different vendors will screw the randomness.
I would order direct from a reputable casino supplies vendor. However given Canasta is non-standard, it seems hard (at least via Google) to find such balls. I'm confused, this is for a Casino here? You should already have a supplier, or ask the manager of another casino.