Pedersoli kits are offered by Dixie Gun Works in either 10 or 12 guage double barrel guns.
These both use precussion locks. The 10 gauge barrels are 30 inches long with the left barrel a modified choke and and the right bore a cylinder bore choke.
The 12 gauge has 28 1/2 inch long barrels with the same chokes as the 10 gauge.
These kits are not cheap. Dixie is asking $465 for either one of them. They also offer these guns in finished condition with checkering on the wrist for $685.
There may be other dealers who might offer the Pedersoli kits at a better price, but I don't know of them.
If your wanting a flintlock, you might consider a Brown Bess or one of the other smooth bore guns.
Of course, they wont mount, point or swing like a shotgun but it wouldn't be the first time in history that they were used for fowling.
They will be cylinder bored and of slightly unusual gauge size. For instance the .75 caliber Bess would be an 11 guage and the .69 caliber guns are 14 guage.