Hi Lulu72.
Welcome to our Mattress Forum! 
SN has mattresses between 8"-14" and I am not sure which model you own or if you have specific preferences or criteria for the cover but here are some options that you might wish to check on …
Latex Mattress Factory (member of this site) has a removable and washable 360-degree zipper cotton cover from 2" to 12" in thickness, which zippers on the bottom.
Arizona Premium Mattress (member of this site) has a large selection of mattress covers, bamboo, organic wool cover, organic cotton cover which meet the flammability standards for their latex mattresses. The zipper is made of Teflon and zips around all four corners making it easy to assemble, but it will only be able to be spot cleaned.
Sleep EZ (member of this site) has Natural and Organic cotton & wool Zippered Mattress covers between 6" and 12" in thickness that zip off and spot clean.
Although this item is a mattress protector from FloBeds (member of this site) it is washable and has wool in it.
There are some other the protector and cover options mentioned in the [url=https://forum.mattressunderground.com/t/foam-factory-any-good]component post here[/url ], but not all are site members.
I hope this helps and that you find an encasement that meets your criteria.
Phoenix