I've been looking for the same stuff for a looooong time now to make a WWII B-17 diorama, but haven't been too successful. However, a few years ago I did find (on eBay) an old Monogram 1/48 UH-1 (B/C????) Huey Medevac kit; I bought it just for the stretchers and figures.
It's a VERY POOR kit, but comes with 2 stretchers w/ molded on casualties and 2 figures to carry 1 stretcher. The whole kit is rough (MUCH worse than the Revellogram Huey Hog kit), the figures and litter cases are well molded but would require some minor to moderate modifications to look life-like or vary the identically stiff poses.
You might be able to still find this kit someplace online or in a hobby store. I think it was re-issued a few years back also, so that may help out in locating one. Hope this helps!