The list of online members here includes a number of manufacturers that make a range of latex mattresses which would be the most likely to make custom sizes but I haven’t asked them specifically so you would need to check their websites or call and ask them. I would also keep in mind that a custom mattress may not have a return or exchange policy and may have some additional costs involved.
The online members that are actual manufacturers with their own cutting equipment that may make custom sizes (although not all of these make latex mattresses) includes …
There may also be some local manufacturers that are close to you that make latex mattresses that can make custom sizes as well and if you let me know your city or zip code I’d be happy to let you know of any that I’m aware of that are close to you.