When I tried Straight Talk a few months ago, the local Wallmart did NOT have just SIM cards. They only had the phone packages.

I had to order the SIM card online and have it delivered.

Since I had a 4S at the time, I also had to get the correct SIM card size. There are adapters to fit the smaller cards into the older phones, and large cards can be cut down to fit the newer phones.

Certainly I would want to have the 3GS unlocked. Not only more flexible in terms of usage, but somewhat better resale value on the iPhone.