Most DJ-oriented cartridges use spherical needles which faciliate heavy scratching and back-cueing, albeit sacrificing a bit of sound quality along the way.
I'll second Sam's recommendation - Ortofon is a brand that's hard to beat when it comes to both scratching ability and sound quality. I've used several models for my own mixshows over the years and I've always been pleased with the end result. Only drawback is that they're not cheap; a pair will set you back $240-$300 depending on which model you buy.
I would NOT recommend Stanton cartridges. Having owned/used several of their needles in the past, they're good for mixing and all, but their sound quality strikes me as somewhat flat/muddy compared to other brands.
Whatever you buy, make sure the needles are indeed spherical - elliptical needles sound much better, but they're not suited for mixing applications (in fact, they'll shred the vinyl if used repeatedly for scratching, etc).
- M