The Camp 9 uses S&W 59xx series magazines. Very common, inexpensive and available from several aftermarket companies. Most of mine are ~15 round capacity. I had a 30 rounder but sold it, because it didn't function reliably. I keep thinking I'll do something with the stock. Don't like what's on the aftermarket, but that's a project for a later date. I want a higher comb and to insure the barrel is free floating. Not into it looking like an AR or a space-age gun. I'm fine with wood.
OH and if you have one (or a Camp 45) you need to get some replacement recoil buffers. The original one will shatter eventually (as mine did). The gun will function without it in a pinch, the problem is getting all the little pieces out of the action/trigger. I think the original tended to be more "plastic" and definitely crystalized after about 20 years. Replacements are cheap and having several spares is a good idea. The delrin ones should last a long time.