Where can I buy a SIM card in the USA?

This is probably a silly Q. But I am not sure, so I ask anyway. I have an unlocked cell phone that currently works more like an iPod touch because it doesn’t have a SIM card. But I have google voice. So I am hoping that if I add a SIM card to it, I’ll be able to use the phone to make/receive calls using google voice and no additional money?

Assuming that logic is right, where can I buy a SIM card? I need the cheapest possible since I don’t plan to be using it for anything but putting into my phone. I don’t want to have to buy a Pay as you go phone along with it. Will I be able to find something to meet my needs from Walmart?

For all those wondering, I already have a proper cell phone that works with Verizon, but I just want to use this other phone to its maximum potential since I have it anyway. Also, the reason I cannot get SIM from Verizon is because Verizon is CDMA technology and this phone needs a SIM i.e. GSM technology

Thanks!!