is it possible...

to find the prerelease packs anywhere with an access key that hasnt been used? the only ones i seem to find are used items from amazon and new ones for $300.

as we get farther and farther from when they were released it will become harder and more expensive to find them. There are a few places that might still have them---small gaming stores and the like. I got my last one (6 years ago) from ebay---it was still sealed and the card had not been scratched....but that was a long time ago.
More than likely your answer will be no.

Oddly enough, my wife just found a factions preorder for my second account at a local thrift store yesterday. Not that I would recommend combing your thrift stores for one, because I agree with coseyfiep. It is very unlikely that you will find any, so I wouldn't go on a treasure hunt for them, but if you are at a place that sells second hand games, keep your eyes open, cause you never know.