Yup, as the others have said, Dario and Rob have some nice stuff. If you can catch an offering from Bill Jacobs (ILikeWood), especially a Cocobolo offering, jump on it (unless you're allergic to it). Low_48 also has some REALLY nice domestic woods.
Those are some of the "regulars" here. Bill Baumbeck has some "eye candy", as does Woodturningz. One of the nice things about Woodturningz is that they will hand-pick through their inventory for something that meets your needs, if you tell 'em what you want. Not that our "regulars" won't, but as a larger business, it's still cool to get that kind of service. CSUSA is another good source for wood blanks, though from what I've seen their selection is more like Woodcraft or PSI, in that the blanks tend to not have too much grain movement or other really neat characteristics. That isn't to say you won't get lucky once in a while, but most of them are just OK. (Sorry Nils)
On eBay, McFamily Hardwoods, as Greg suggested, has a nice selection, though I've never purchased from him. Chitswood has a great selection of various woods, including a really nice pen blank sampler pack that you can pick up pretty cheap. The figuring in the sampler isn't always all that amazing, but they are usually nice blanks and you get a wide variety of stuff.
Is that enough to get you started? If not, I can give you some other sources too.