I use ebay, as one can find, in my opinion the best deal, even on rare stuff, and many bigtime sellers will offer some kind of satisfaction if youre not toally happy with it.
I just got two today from a guy named
skamaninbelfast and he has a pretty decent selection of older, harder to find numbers. I was able to combine on shipping too, which was nice.
one guy,
feral100 has a good selection of stuff he claims is new, and although he uses stock photos, he has an excellent rep and all those who have recieved have raved about him.
A third one who has a good selection of more recent, but still very cheap stuff (every shirt starts at 2.99 pounds, or about 5.35 USD and most dont even get a single bid) is
mattowls
luckypenny2003 has some stuff that is worth a look, but it took me forever to get it (might have been no fault of his, but you know...)
Hope that helps