There is one on the list:
S03344 20 Ga., 28" Vent rib, Screw-in choke (Mod) $79.00
You can also get the 22" choke tube barrel from the 20 Ga Youth model Pardner (SB1-25C). For turkeys with a 20 gauge, that's the one I'd want. It's not on the list., so you'll have to ask customer service how to order it.
With either barrel, I'd order without the choke tube. That should save you around $20, and you can use that to buy a better choke tube than you'll get from the factory.