I always seem to have trouble finding prepaid SIMs in the first place when I travel. You'd think they'd be easy to find in airports, especially in international terminals, but I've wandered around airports in Australia, New Zealand, and Iceland without having much luck finding them.
In Melbourne I did eventually find an electronics store while wandering downtown and they sold me a prepaid SIM, which I was then unable to activate because the carrier wouldn't accept payment from a credit card associated with a non-Australian address. So I had to get my friend who lives there to pay for it with his card. It was ridiculous and a huge waste of time.
These days I'm happy to avoid that trouble.