100% Sodium Hydroxide? Didn't know you could get that in a hardware store, let alone legally dump it down the drain =P Will have to remember that.
If I'm not mistaken another good choice for electrode material is graphite it's highly insoluble, just don't try to use mechanical pencil lead, most modern varieties use a polymer graphite blend that makes it totally unusable as an electrode but keeps them from breaking as easily.
Another good anode material is graphite. If you're using stainless here's an idea, use a mesh bag to hold a large quantity of stainless steel threaded screws/nuts/bolts as small as you can find and use a single rod down the center as the electrical contact. This is how the anodes of most plating operations are constructed and it allows for extremely high surface area.