Caswell sells 8"x 8" lead plates listed as a GP plate or anod plate under Anodes in plating supplies.
http://www.caswellplating.com/kits/access.htm#anodes
The spots usually show more in cast aluminum from a poor mix in the cast when it was poured or high level of contaminates like silicone. Sometimes you'll see them in billet if the part was machined aggressively and developed hot spots in the material. The method to get rid of them varies with the part. Sometimes just stripping, buffing, and reanodizing will do it. Etching helps sometimes. Acidrain does alot of paintball guns. He can probably tell you just what you need for that piece.