Where can I find a copy of Betrayal at House on the Hill
The game is not currently in print; so the only copies that exist are ones that were printed before, and people have bought.

Unless you get extremely lucky, and stumble upon a game store that has a random copy sitting around that hasn't sold (not very likely at all), then the only options are to go online to places like Ebay, BGG Marketplace, Amazon, etc, where you will have to pay an increased price because of high demand.

You can follow this thread for updates and details; it seems like they are keeping that up to date with info about a possible re-re-release (it was already re-released once).

Also, keep checking constantly online! Check Ebay and Amazon at least once a day; every once in a while you get lucky and find one for a reasonable price.