Whilst some steel is marketed as stainless....it has more chrome or other metal content...no steel under the "wrong" conditions is ever truly rustless. Indeed highly polished steel...such as a ball bearing...has its own rust resistance. Just through being highly polished.
Most steel shot is, or used to be, IRON not in fact steel.
So a "cheap" rust resisting steel shot might be simply high quality high polished ball bearings. As to its suitability for use I wouldn't venture an opinion. Gold plated soft iron shot might be a cheaper alternative.