AFAICT, the answer is that you don't buy an IPv6 address.

You're not even supposed to buy an IPv4 address, but a market for IPv4 addresses sprung up due to their scarcity.

Now, I'm not trying to be pedantic here. To obtain an IPv6 address, you need to be allocated one. And that's the job of your ISP, apparently. Which kind of makes sense since 1) that's how your home computer gets an IPv4 address (a dynamic one, these days), and 2) unless your ISP supports IPv6 you'll need to set up some sort of IPv6-over-IPv4 tunnel anyway.

Now, you may say "oh dear god are there any options that don't involve my ISP", and, well, good luck: https://www.apnic.net/get-ip/get-ip-addresses-asn/

By default when you're allocated an IPv6 address you get at least a /96 block and thus you'll never run out of sub-addresses, so you really only need one.