I have looked in Walmart and they seem to only carry to size 6
Being a relatively low volume item, the size 7 diapers are sold in very few places, namely, Toys R' Us, and Babies R' Us. They're also typically sold in larger bulk packs, so upfront cost, and stashing them away may be an issue.
If you can here is a way to buy them online for you online folks
you can also have them sent to your nearest toys r us or babys r us