New gun!
I had an old S&W pump retired police car gun... would it have likely been a 3000?
This was 10+ years ago and it was long out of production then so I traded it for a S&W .38 wheel gun.
Brent
The only possible source might be Gun Parts Corporation.
They bought all the S&W parts, and did sell original mag extensions, folding stocks, and other police accessories.
Note that after S&W dropped the Model 3000, Mossberg picked it up and sold it as their Model 3000 for about a year.
Gun Parts has parts under both brands:
You'd have to contact them to see if they might have one lying around that's not listed on the site.
Other options are to have a custom extension made up, or to buy a genuine Remington factory extension kit, and have a machine shop make up a new coupling that will fit the 3000.
I "think" there's someone who makes custom extension, but I can't remember his internet ID.