Galazan may be better known as Connecticut Shotgun Manufacturing the maker/importer of the reproduction Parker shotguns. They have a fairly extensive selection of parts and a gunsmithing department, Griffin & Howe may be another place you could check.
Galazan aka CSMC was neither the maker or importer of The Parker Reproduction shotgun. They had nothing to do with that particular shotgun. Jack Skeuse, the owner of White Flyer clay targets originated the idea and had it made by the Winchester facility in Japan.
https://shootingsportsman.com/parkerrepro/CSMC currently makes the Model 21 and would be the first place I would look for a Model 21 part, if having difficulty finding it used.