Well, I'm not sure what type of custom decks you're looking for. Whether you're looking for already made custom decks or are wanting to make you're own. So I'm going to post two great websites!
Okay, this is what I think you're talking about, so I'm going to post it first.
BMPoker World:
http://www.bmpokerworld.com/
That's a great website that holds tons of cards! 62 different types of Bikes! 62! And that's not the only type of card they carry.
62 Types of Bikes, 6 types of Bee, 43 Casino, 12 Novelty, 8 USPC, 10 Tally-Ho, 4 KEM Playing Cards, 6 World Poker Tour
Now, if you're looking to make you're own custom deck, here's a great website!
PFI Playing Cards:
http://secure.card-press.com/PFIPlayingCards/tabid/133/Default.aspx
Now, I haven't ordered any cards from them. But, I've heard from other people that they're pretty good quality! Plus, it's the only website I found where you could order 1 deck! 1 Deck = $14.95 though! But, it's a great place to make you're own cards!
Hope that helped you out. Peace!
Tyler