(Disclosure, I may receive a small referral fee when you make a purchase through links on this post.)
Google announced at their breakfast event today that you can purchase the Google Chromecast online and at brick and mortar stores. As we stand right now, both Google Play and Amazon are all sold out. If you missed the stock earlier, your best bet would be to call up a few Best Buy like this lucky fella.
Screen shot of Amazon page for Chromecast
At the Google Play Store, it says the item will ship in 3-4 weeks!