I've also been looking for blank backed decks and the cheapest I've found has been
http://www.cards4magic.co.uk/ - thanks to Mandrake for putting me on to them. Other places are charging £8 for a blank back deck, c4m are charging £5.50.
Great service too. Ordered a few decks from them, came quickly. Had a manufacturing problem with one of the decks (not their fault) - they actually sent me a replacement before I posted the faulty deck back to them. And they gave me money for the postage too.