RMC sports.com has touch hole liner replacements with the allen head part# ML187 and states they are a M8X1.25 threads for a Pedersoli rifle. I believe this is the one you would need. You can go to their site and search by the part number or the name in their catalog section.
Cayugad is right. That's the one you need.
But the RMC site is not very friendly and has no search function. Go here
http://rmcsports.com/catalog.htm and click on
Black Powder Shooting Accessories on the menu at the left hand side of the page, then scroll down to the
Nipples, 209 Adapters, Touch Hole Liners section.