contact Hakhel, they have a list of various gemachs including medical ones, which may have a portable ramp or tell you where to buy/rent it. GEMACH LIST – send a self-addressed letter-sized envelope with two first-class stamps to: Hakhel List, 1327 E. 26th Street, Brooklyn, New York 11210-5204. or call 718-252-5274 for more information
also, when we were looking for such an item (we didn’t end up getting it, though) we did actually see it on “carol wright gifts”, but i can’t vouch for its safety rating.