Try the local saddle & horse gear shop. Ours (in Laramie, WY) has a big rack of rings, dees, buckles, etc. In fact I bought some plated rings there to make oar keepers, which haven't rusted a bit in 5-6 years of use. (Horse sweat really corrodes cheap plating, so the decent suppliers stock quality metal parts for harness and cinches).
Stainless steel rings likely cost quite a bit more. You could look at sailing suppliers: SailRite, MauriPro Sailing, Seattle Marine. Or just google stainless steel ring and see what pops out.