I don't know where you live so I can't help you there. Anyway where I live the only place I saw Oblivion and Fallout 3 GOTY was at EB games which is basically Gamestop. You may find it at Wal-Mart, The Source, Radioshack, or Future shop if your lucky. Also try bargain stores and flee markets. I've found brand new sealed Morrowind GOTY at Bargain store for $5.99 and brand new sealed Oblivion Vanilla for $10, Fallout 3 was not there though but it is possible.
Local stores may not be an option. I live right by Microsoft and Nintendo and Digipen HQ and they edge out all of the competition, and none of them carry Bethesda's games.
I'm probably gonna buy it online. New Egg has worked for my brother before with New Vegas.
Both are reliable retailers, just go for the cheapest. You won't have any unusual problems with your set up, just the occasional crash due to the game engine not handling multi-core processors and multi-threading well but they aren't frequent enough to worry about.
Also look at a eBay and sellers with over 1000 sales and 99% rep.
Also 2: Install at least the Broken Steel DLC at the beginning, otherwise the ending is very disappointing.