Browse your local yellow pages telephone directory for electroplating shops. If you are near or in a major industrial area, you should be able to find a shop that does electroless nickel plating. Specify that it is for a part of a gun, and not the whole gun itself. Federal and state laws require an FFL for working on receivers or complete guns. They willl tell you to strip all the parts off and bring in the bare slide.
An alternative of course, is to ship it to a gun refinisher but that entails shipping costs and waiting time.