i found a great site called newtsgames.com, they not only had the DB decks very cheap, but a lot of other decent ones as well. their selection is definitely not the most vast, but i did find quite a few nice things there (raiders, tragic royalty)
They're one of the retailers that the bicyclecards.com website directs you to for making purchases. They're a standard retailer - unlike the aftermarket retailers who deal in collectibles, these folks deal in new product. When a sold-out deck is in stock and has been around a little while, they mark it DOWN, not up, to clear it out faster and make room for new merchandise. They have the Bicycle Raiders in white at half-off, only $5!
If your supply of the David Blaine marked decks runs out, you have alternatives. A pair of French magicians created "the Ultimate Marked Deck," based on the Bicycle Rider Back, and it uses an ingenious marking system, less detectable than the Blaine Marked, but just as easy to read. It's out-of-print and can no longer be reprinted on a Rider Back, but because of it's retail price (generally between $30-$35), many places still have some in stock. Someone else created a "Speedreader" marked deck on Mandolin Backs - the marks are on the right corner instead of the left, making them harder to spot. And there's the decks from card-shark.de, the makers of the Phoenix Back. They have numerous models of marked decks, reasonably priced when compared to the rest of the market, and specifically designed for magicians as well.
But a $4-$5 marked deck like the Blaines is tough to come by, especially at that level of quality.