Put questions like this in the SS/FG subforum... You would have been told 5 times within 3 minutes of your post: "BB lockring".

Old bottom bracket lockrings used in loose ball BBs have the same size and threading as the hubs, so they spin on nicely. About 1,5-2mm thick, I guess, should be under $1 or free in your lbs. Make sure that the lockring still has enough threads so it doesn't strip.