According to the webpage in the Google Play store, you can "buy them at your local retailer."
Google Play gift cards are - at least for now only available from Target, RadioShack, and GameStop. Nope, not available at Best Buy, and apparently not available for purchase at those retailers' websites, either.
The cards come in $10, $25, and $50 increments.
Don't buy them as a way to subsidize a friend's Nexus 7 purchase, though. The gift cards cannot be be used for hardware and accessory purchases, Android app subscriptions, or magazine subscriptions.
