Have you tried Nations Lab? I use ACI lab (American Color Imaging) for a lot of my products, but not for one that large . . . I'm sure they have that capability.
I wish I could help, I ended up buying my own large format printer a few years ago because of things like this but I don't reccomend that option unless you find a used one in decent shape at a reasonable price.
I've had a nice 12x36 done at Costco. And since their standard format was 24x36, they printed 2 on the 24x36 for 13.95$. Quality was great and so was the price.