How to buy games on ps4 store american

So i live in new zealand and i have made a american account so i can have access the american store/games. The only problem is that my nz debit card does not work on the anerican store. I have tried purchasing a $20 card anerican card from sites like amazon and best buy but they won't let me as i don't have an american card i imagine. Can anybody help me out? Thanks