BluegrassOz,
I don't know of anywhere that sells sim cards. We get them at the store for the carrier that the phone operates on. Actually, I don't even think the stores will replace sim cards on phones that are not part of their model line (i.e. phones they don't/didn't carry). Also, i've never paid for a sim card. They replace those for free here (since they're getting our money for the monthly plans).
Since it appears you have one of the non-carrier specific phones, I don't know if international calling is an option. If it is, you can just add international calling to your plan (added charge of course) and then just remove it when you get back. Be warned .. international texting is EXPENSIVE if you don't have it on your plan. I got a $400.00 bill due to my daughter sending texts from a cruise ship last year. So do your homework.
You can get one of the non-tethered phones when you get here, but, I don't know which ones have international calling included in their plans. The link below has the pre-paid phones you can get at Walmarts here. I've heard good things about the 'Straight Talk' phones .. unlimited voice, data and text for $45.00/month (US). Note: that rate is only available online. THe monthly purchase cards in the store will be a bit more.
Shop around a bit, I'm sure you'll find some good deals for your needs.
Here's the phone link. You may be able to order it and have it in your hot little hands before you disembark.
Best of Luck,
Kevin
Listening to:
Lyle Lovett -- Up in Indiana (From TBF Tapers)