Sources for Factory extensions:
Direct from Remington.
Have your local dealer order one from a Remington factory authorized Law Enforcement distributor.
Ask here:
www.capoliceequip.com/And here:
www.impactguns.com/store/Order from Brownell's
www.brownells.comWatch Ebay for a used one.
Look in The Shotgun News for distributors who sell Remington Police guns and order from one of them.
For other makes, there are:
Choate:
www.riflestock.com/Wilson.
Vang.
Tank's Rifle Shop.
Tac-Star.
The best unit of all is the Remington factory.
The best all around unit is Choate from the stand point of quality, price, and availability.
The Choate comes as a complete kit, with the extension, spring, stronger follower, sling swivel, and the all important barrel clamp.
It's available in more sizes than all the others put together, and comes in black or parkerized.
The parkerized matches the Remington Parkerized finish.
The Wilson is good, and EXPENSIVE. It does not come with a clamp.
The Vang is good and expensive. Also, no clamp.
https://www.vangcomp.com/accessories.htmlThe Tank's Rifle Shop version is a close copy of the Remington factory. Good, and expensive.
It's just under the Choate model:
www.tanksrifleshop.com/remshotgun.htmThe Tac-Star usually doesn't come with a clamp. They have some past history with quality problems like out-of-round magazine mount collars or tubes that are out of line.
Tac-Star is available in parkerized.
Whatever you get, GET A CLAMP.
It doesn't matter if the extension is a solid steel bar, it's still screwed onto a thin magazine tube, and a good bump will knock it off the gun, damaging the gun's magazine tube.
This means a trip back to Remington for an expensive replacement.
If your extension doesn't come with a clamp, the best one is Uncle Mike's close copy of the Remington factory clamp.
www.unclemikes.com/adtemplate.asp?invky=9004490&catky=1465723&subcatky1=4548843&subcatky2=2000180