To reply to you and the parent comment. I did check a few stores that said they made custom sizes and a lot actually wouldn't go up to 7x7, although it's clear from your link that select-a-bed does.
It wasn't so much price (although the fact that selectabed asks you to call for a quote suggests they're fairly pricey), it was that once I researched mattress materials, I was pretty set on having buckling column gel as the top layer.
I don't blame you. I tried a purple mattress (which I had no idea wasn't a new or unique "technology" until your post) and Im pretty sold on getting a similar style mattress now.
Another option: https://www.selectabed.com/oversized-king-ultrabed-mattress/