Thanks for the link. As it reads, "2 x 1.5V DDR3 DIMM sockets supporting up to 8 GB of system memory" and "Support for DDR3 1333 (O.C.)/1066/800 MHz memory modules", I'd simply try to find a pair of 1.5V 1066 MHz DDR3 modules of 4 GB each. Or, as those are hard to find, a pair of either 800 or 1333 MHz ones, possibly underclocking the latter to 1066 MHz. Usually the higher speeds modules are automatically set down to the nearest supported speed. The amount is more important than the nominal speed (not saying that speed won't matter, though) as long as we're within the common speed range of that type. A pair is the critical feature in my book, i.e. identical sticks. Kingston even sells matched kits which may or may not be a marketing gimmick.
Ebay is as good as a buying place as any other web shop. It allows for smaller vendors to find buyers for items they've bought more than the local demand was, also you can find items that are long sold out from the manufacturer's stock. Of course you'll have to be careful but there's ways to secure your payment in case you get a lemon or worse.
