Quite rare, $200 or something to buy one it seems. PC Gamer gave away 350 of them supposedly, but they've been hit by bots the past giveaways they've had.
http://www.pcgamer.com/2013/09/19/he...-key-giveaway/
EU just had another round of opt-ins yesterday I think, but nothing seems to be planned for more opt-ins in NA (opt-ins meaning Blizzard sending out keys to people who've got battle.net accounts that have indicated they wish to participate in the beta).
Don't know anywhere else that's giving away a significant # of keys, some social media guys are getting batches of 25 to giveaway on their videos or whatever, but a very large # of people want keys...
Play GTAV until open beta I guess.