Being of Scottish stock I am rather frugal (a.k.a cheap) so I buy mill finish stainless steel plate or bar and finish polish it myself. It is not difficult, just time consuming.
You get a "1/4 sheet size" finishing sander (one of those with a square pad that takes 1/4 sheet of flat sand paper) and then start with either 80 or 120 grit sandpaper until the major ridges, valleys and marks are gone. Then you work down the entire range of sandpapers until about 400 grit. Then switch to automotive finish sanding
paper in grits from 600 -> 1000 -> 1200 -> 1500 and finally 2000. By 1500 and then 2000 grit paper you will have a mirror finish on the stainless steel. Starting with the automotive finish paper I use a sponge with water to keep the surface clean and cool.
Passivation is merely chemcially removing the ferrous ions/particles and leaving the chromium behind. This can be done with Nitric Acid which is not generally available so I use Wichard Wichinox. If that is not available there are other "polishes" that have "acid" in them and claim to passivate Stainless.
Inspecting plates I look for surface pitting or cratering that looks like something is "eating" into the metal. This is the telltale that the corrosion has started and the plate is probably a goner. If that is not there then polish up the old plate and inspect it again. If it is shiny and smooth put it back.
I have found the key to successful drilling of holes in stainless is a drill press with very low speed, cutting oil
, and "cobalt" drill bits that I get from Home Depot or other tool shop.