It depends on what you're looking for. Kemin Hu's website is wonderful (spirit-stones, as referred to by Harry), but they are not traditional suiseki. She is mostly selling what would be classified as gongshi (scholar's rocks). She does have wonderful stones, and there's usually good deals during the holiday season.
If you're looking for Japanese style stones (suiseki), then Sean Smith would be a great resource.
Finally, you could by from Chiara in Italy. Although I have no personal experience with purchasing from her, I do know some that have, and they have reported positive experiences (in fact, it was a friend of hers in Virginia, he's a well regarded stone enthusiast and I trust his judgment). Here's her website: http://www.padrini.it/zen/shop-en.htm.
The stones are mostly (guessing a little) ligurian limestone.
So, I guess it all comes down to what you want.
Best of luck