not mail order no, but power armor like that does exist today https://en.wikipedia.org/wiki/Powered_exoskeleton
So it is not beyond reason that something designed for persons with disabilities might be... overclocked... into something like what was in the Manga, not to mention that there ARE military examples in the US and Russia, at least in the proto-type/pre-roll out phases.
So again, I do need to say that I think that people are more accepting of magic in Ranma's world then average.