if they are black and white then they are ocellaris. There is no such thing as black and white percula although that is almost always the name they use when selling them.
edit: polymnus (saddleback) clowns are often sold by the name black and white percula too. Black and white ocellaris are usually much more expensive than other clowns while polymnus are about the same price as others. Around here polymnus are around $20, about the same price as normal colored, captive bred, percula while black and white ocellaris sell for $40 - $50.