Q: how can i purchase a TV series from the american itunes store when I live in canada?

You can only purchase from your own country's store - to be able to buy from another country's store you need a valid billing address in that country, and be in that country when you buy from it.

You can try requesting that it be added to the Canadian store, but ultimately it's the TV company that will have to grant Apple a licence to sell it there : http://www.apple.com/feedback/itunes.html