+1 for Beatspace, they often stock things long after everyone else. If you're looking for something distributed by Wirikuta (which is a hell of a lot of stuff) then their website is a good bet. For downtempo, I've found that the inSpiral shop sometimes has things that are hard to find elsewhere. Same goes for Farfield Records. When all else fails, it's a good idea to check the label's site in case they have their own shop (Avatar do, for example).