Academy's M977 HMMWV Ambulance comes with a couple nice ones. Italeri's old Jeep Ambulance had a couple open and a couple folded ones. Tamiya's British Land Rover Ambulace has some too, and a guy on a litter. The Revell and Academy MASH Bell-47 helos have litters. Some resin sets have them too.
They are actually pretty east to make. I made folded ones for my UH-1H Vietnam MedEvac helo. I used a couple stryrene rods for the side poles, strip styrene pieces for the support bars, bent some strip styrene bent into "squares" for the feet, and lead foil for the cloth.
If you can go with 1/32 scale, Fireball Modelworks makes a couple sets w/folded and open ones.
Folded as part of his UH-1H MedEvac interior set (he will sell them separately).
Open litter set.
Good luck.
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