www.aebike.com look in component catalog under bottom bracket and at the YST offering. $10, probably a screw in replacement unless you have Italian or French threads, in which case check out Harris
Cyclery (under Sheldon Brown). Next cheapest is Shimano Sq Taper sealed bearing drop in eg UN54 at
Aebike.com though these seem to all be plastic cups now ($24). The YST will work with your present axle most likely.