So by now I think just about everyone with a Steam account has received DOTA 2 early access for free (it sells for $29.99 in the Steam store), but if you haven’t you can find a Steam Key quite easily. Valve often sends Steam users bundles of DOTA 2 keys to send to friends so that more people can play the game, and I recently received 10 keys to give away (now 8 keys as of this posting).
If you’d like one simply add me on Steam (Darth Gumballs) and drop me a message and I’ll be sure to hook you up with a DOTA 2 key.