I usually buy bikes from Costco, but I used to get them from eBay. You can get a brick for £20.85 including P+P (which is £1.74 a deck) here: http://www.ebay.co.uk/itm/12-PACKS-BICYCLE-PLAYING-CARDS-BRIDGE-POKER-BRAG-BIKE-/180763405068
These are of course the new "Bicycle Standard" decks so...