For the Graphene OS AOSP fork, these are the compatible phones: https://grapheneos.org/faq#device-support

You gotta make sure to get it "unlocked" so no buying a phone plan during the purchase.

https://store.google.com/category/phones?hl

Then follow the web installer instructions https://grapheneos.org/install/web

Alternatively, you can do the don't install it yourself option (which is inherently sketchy and inherently opens you up to abuse) by buying a preinstalled one.

https://liberateyourtech.com/product/buy-grapheneos-phone-pixel-new/