Looking to buy Amazon gift cards using United Airlines miles

Posted in r/giftcards, thought it wouldn't hurt here as well.

So, I recently started to accumulate United Airlines miles from work. I noticed that you can actually redeem them on gift cards, but Amazon isn't an option. That said, I wanted to know if anyone out there has found a successful chain of other gift cards to purchase to end up with an Amazon one. For example, could I buy a Walmart card and buy an Amazon c a rd from Walmart? A lot of stores don't allow this, and not even every store sells Amazon gift cards, so I figured I would see if anyone knows of a gift card chain that actually works. Thanks!