You can buy replacement inserts from Hilts/Dolphin, at least you could 3 or 4 years ago.
Hilts Website
If anyone is interested, I have two of the Hilts bullet sinker molds that I would sell:
LMBULLET Mold: 1/8 oz., 1/4 oz., 3/8 oz., and 1/2 oz. cavities; has one wire that needs replaced; $17.50 + $4.95 shipping
LMBL-M Mold: 1/8 oz., 3/16 oz., 1/4 oz., and 5/16 oz. cavities; $20.20 + $4.95 shipping
If interested, Email me