Anybody know where I can find a mag extension for a Benelli 20 gauge???

[SBE2]

Headed to South America again this spring/summer for doves, pigeons and perdiz. Very interested in locating a magazine extension for my two Benelli 20gauge shotguns. I was able to find a 12 ga mag extension at Cabelas last year, but have not been able to find a 20 anywhere. If anyone knows where I could find them I would really appreciate a heads up. Already posted in the buy/sell, not having luck anywhere and looking for help. Thanks.