Need a pre-paid micro sim for my iPhone

Going to the U.S. in a few days. I need to get a pre-paid micro sim for my unlocked iPhone 4 to use while in U.S. Wondering if I can purchase one at Lakeside or in Guad before I go. Any specific help would be appreciated.

I have only found them in the US. Walmart had one for around $40 with unlimited voice and data for a month. It was cheaper to bet the month even if not needed than to get shorter time periods or a phone with sim and minutes. The one I used had regular and micro sim cards included.