You'll pay through the absolute nose for a bespoke mat in a specific size.
I'd go larger and tape it off, or there are some "almost" sizes. Games Workshop arbitrarily decided on a new size for their main games at 44"x60"? That's closer than your normal 6'x4'.
The failed FFG games used to use a weird skinny mat which was...(googling)...6'x3'...but most shops have given up making those. It was used for their failed Rune Wars and maybe Star Wars Legion (though I don't think people care too much about "specific" size mats for their games.
So you may want to look up Rune Wars sized mats? If you have issues cutting a mat, you could take it into a shop where they sell fabric and see if they have a precise way of cutting it for you. I know when they cut yards of normal fabric they're not going it very precise ---- but perhaps they have a system. Or...failing that, use tape to make a fine line and have someone else cut it with a nice new pair of scissors (Fiskars or something).
As above - I'd just get a 44x60 and let the extra hang off. You can always use less of a mat, but you can't add more on.