Where can I get Portal 2?

The best place to get it from is probably Steam (store.steampowered.com) as they're always having sales on there. In fact, if you're prepared to wait a few more weeks they'll probably be having their summer sales by then and you'll most likely be able to get it really cheap!

If you want a physical copy of the game, you may still be able to get it in some high street shops. It's available on Amazon as well.