For reinforcing thumb break tabs I sew a small piece of 22 gauge sheet steel between two layers of leather. The steel that I use is just regular mild sheet steel from Menards that comes in a 24"x24" sheet. It's thin enough to cut with metal shears but heavy enough that I haven't had to worry about bending it through everyday wear and tear.
No matter what you do, you can't go wrong with stainless. If you're worried about rust prevention with mild steel, you could always spray it with metal primer before sewing it in.
Here's a couple photos of the finished project; one of the holster itself and one of the thumb tab. Wish I had a few pics to share of it while it was in progress but I didn't think of it at the time.
