The cartridge ceramic bearings for BB would normally have corrosion resistant races, see
here. Enduro also offer SS bearings that are a cheaper option than ceramic. Enduro resellers, also on Ebay, may offer lower prices than the manufacturer.
Corrosion 'resistant' isn't corrosion proof, just like water resistant isn't water proof. And you don't think stainless steel is corrosion proof do you?