Hard chrome is probably the closest thing to stainless, appearance wise.
Other choices are baked on silver paint finishes, like Gunkote. Or some version of nickel, which is a metalic plating that doesn't look alot like stainless (I don't know why this would be prefered to chrome). And lastly, a Glock slide, being corrosion resistant due to its tennifer heat treatment can be polished until the black oxide is gone and the bare steel shows, but it will not corrode. The other guns could probably be treated similarly, but their more complicated shape makes it harder to do, plus the HKs and Walther are available with stainless slides.
Really, hard chrome is definitely your best option, but if not, the silver Gunkote is probably the least painful option.
Just a slide will run between $30 and $80, for any of the processes, FYI.