moo.com after they did a promotional tie in with Flickr. I'd always threatened to get more because I loved the quality and the mini card size.
Well, good news, MOO has opened a US printing facility. So if you live in the States your order can be sent in five days.
I highly recommend them if you're looking for quality cards. The added plus of having them be as individual as you want them is an amazing touch. 100 quality cards for $20 (plus shipping.)
Give them a shot.