Another vote for the Mouse Factory, because they can have your order to you the day after you pay! They're really great. I've never gone anywhere else to compare, but that's probably because they're so good.
Also, once they accidentally sent me 100 large pinks instead of 50small and 50large pinks. On my next order they threw in the pinks for free, except since I didn't really need the small pinks anymore they gave me large ones. How's THAT for great customer service!
Another time they over charged me maybe $10 or so, I think it was something like the shipping ended up not being as much as they originally quoted.. so what they did was knock off the $10 from my next order.. and my next order was maybe.... I don't know 6 months later. They still remembered and stayed true to their word!