John is correct. Almost all clock supply companies sell weight inserts for shells. Both cast steel and lead. Pick the one that is closest to your location as shipping is where you’ll pay the most. All shippers charge by the pound so, 2 or 3 weights @ 8 – 10 lbs a piece can be pricey. They should also carry long case weights as well.
Timesavers is good for central / west. Clockworks is good for east.
Good Luck and happy hunting!
Mike