Sony's memory is, as it has been since the beginning, hard to find at a discount.
I prefer getting used sticks on eBay, from either the Best Buy Outlet or one of the dealtree-based sellers, as long as I can be reasonably sure they are official Sony product. Avoid Hong Kong sellers--even if they swear "100% official", you'll likely get a knock-off with a higher chance of failure.
I also happen to have a spare 8GB Pro Duo Mk. 2. If you want to go that route, check my tradelist for more info.