First up, have you used either before?
Personally, I like both brands. Sanwa sticks are really, really loose feeling which puts some off. Persevere with it though, and you'll find it incredibly accurate. This is my preference for VF4, T5, etc. Seimitsus (except for one model, I think it is the LS-33???) are tighter sprung than Sanwa. You can feel the switches clicking more clearly and it doesn't feel quite as smooth. People prefer Seimitsus over the Sanwa JLF for their shorter throws and smaller deadzones. Only exception to this rule is the LS-55, which has a longer shaft than other Seimitsu models and therefore all stick movements get exaggerated by the extra length. I actually managed to remedy the wider throw of the LS-55 by sticking a second square gate over the existing one, so it isn't that much of a problem. I tend to use Seimitsus for 2d fighters, where for some weird reason I have problems executing supers in the likes of SF and KOF with the JLF. Where shooters are concerned, Seimitsus tend to be preferred, but I've had to make do with using a JLF and have had little problem.
As for sellers, I hear some fella called Ponyboy on SRK sells Sanwa stuff.