How do I activate my itunes gift card without a sales receipt?

I bought a $100 and 2 $15 itunes gift cards at Walmart and something weird happened at the cashier and had trouble activating it. So I picked a new one and bought them for about $135 including tax. I don't have the sales receipt anymore but when I tried redeeming these cards only one of them worked. One of the 15 dollar ones while the 1-- dollar and the 15 dollar one said they weren't properly activated. When I tried getting a refund they told me to ask the company. However I don't have the sales receipt and i'm not sure what to do.