If you're wanting drum samples, consider the
Bad Ass Snares Bundle, a pack I use regularly for all manner of snare drums.Generally buying anything with a dubstep label on it is a bad idea, it means they're pre-processed, will be unsuitable for other areas, and will be difficult to get to sit in your mix. Places to look would be Beatport, Hospital Records, and if you want what everyone else is using, Vengeance, obviously.
If you're planning on buying a preset pack, be very wary of what you'll be getting. Often the sounds are specifically tailored to the demo tracks the company produces, and while impressive sounding, they can be near impossible to implement in a mix without knowledge of synth design. You'll never be satisfied if you don't. They can make a good starting point, but nothing else.
If you're planning on buying a pack of pre-recorded dubstep bass sounds that you just drop in your track... you're wasting your money. Get a VST instead.