Best Place to Buy US Google Play Cards

I'm new to Android, and I've set up a US-based Android account so that I can run US apps (Netflix, Hulu). Now I'd like to buy some Google Play credit so that I can buy stuff on the store, but I don't want to give Google my credit card.

Since it looks like Google Play credit is region-locked (well, currency-locked), that means I need to buy Google Play credit in US dollars.

So, where's the best place to buy US credit? I had a look on eBay, but they make vendors ship physical cards, so $100USD worth of credit costs almost $150AUD including shipping, which seems a bit steep.