Lyman used to list spare parts on their website, although I have not checked recently. I seem to remember a drop-down menu keyed to the part numbers on an exploded diagram. It would be worth checking.
Dixie carries spare parts for some production guns. I don't know about that one, though.
I have a Lyman Plains Pistol in .50 caliber. I've been thinking of having it bored out to .550", which would be a tru 28 gauge, so pre-cut wads and shotgun cleaning tools (jags, brushes, etc.) will fit. I keep talking myself out of it, though.
Best regards,
Notchy Bob