If you mean the 26 inch S5 (ISO 597mm), the only new choice is the basic Kenda 3-speed type tire. New old stock tires sometimes come up, but they're basically like 3-speed types and not particularly fat. If you try converting to British 590mm size (26 x 1 3/8) you have a few more options, but you need to check caliper reach to verify it will fit.
If you mean a 27 inch rim (as are found on 10-speeds usually), you have the Panaracer Pasela as a somewhat larger tire, but it is not truly fat either. Schwalbe also makes a couple tires larger than the skinny types, but they're also not all that "fat" in the scheme of things. You get more tire options by converting the 27 inch wheel bike to 700c. You'll need to verify caliper reach if you decide to do that.