+1000 on Havlinsales.com for anything Mossberg. Victor Havlin and his wife Cheryl run the Mossberg Collectors Association, and are real decent folks to deal with.
Your shotgun is (I think) a Mossberg 390. Vic does not list a 16 g C-lect-choke, but email them and ask. If they have one, will probably run you $25 and shipping.