Hi AT,
I haven’t talked recently with the different vendors in the component post here about all the cover options they currently have available (websites are often not up to date) but a quick glance shows me that you have a few options:
Sleep on Latex makes covers larger than 6" in house as a custom order. They may be able to do something larger. You’d have to phone them to find out.
Sleep Like a Bear offers custom sizes.
DIY Natural Bedding offers a kit where you can custom order extra fabric for thicker sizes. I don’t know if they would make one for you themselves but you could call and ask.
Foam Order can create custom sized covers.
MattressCover.com – you have to call them for their complete selection.
This listing is of course by no means complete but it should give you a good start.
Phoenix