If you can't find one factory made, several companies do an excellent job of installing RemChokes on an aftermarket basis. I have always used Colonial Arms in Selma, AL, they are a home-town company for me and have done excellent work on every barrel of mine they have modified.
http://www.colonialarms.com/
Briley has an excellent reputation with the competition folks...
http://www.briley.com/
Nu-Line has been at it for a long time too- so has TruLock. Here's a list, partial anyway:
http://www.shotgunsports.com/choketub.htm
Call whatever company you decide to use, ask them about necessary measurements before installing tubes. There needs to be enough 'meat' in the barrel walls to allow installation of factory tube sets, better to measure it yourself before you send off your barrel.
While you are at it, have the forcing cone extended a bit in your barrel too. That seems to be the most consistent mod that has a positive influence on patterns.
Now that there are some new developments in low recoil buckshot loads producing excellent patterns out of more open chokes, you might want to reconsider the whole notion- at least do some experimenting beforehand with the new ammo. Installing choke tubes and getting the forcing cone done will cost about $125 from Colonial (maybe more now) and likely a lot more than that from some other sources. Remington has been producing most of its factory short barrels with fixed IC chokes lately, it may be that's enough choke for current applications. It has been for me of late, I had rather just buy the IC barrel and be done with it, and get a $30 forcing cone job done on it when I'm in Selma.
lpl/nc (getting 6" or better 00 buckshot patterns at 25 yards with more than one 20" barrel with a MOD RemChoke tube and forcing cone job)
