Contact Jess Briley Mfg at: http://www.briley.com/
but don't expect it to be cheap. They were one of the first to do this type work and have an excellent reputation for quality. I use their chokes in my Browning O/U 20 gauge. I'm curious. Why cut all the way back to 18 inches and then install choke tubes? Typically 18" guns are for defensive purposes and then you don't need chokes. But if you want chokes, why shorten the barrel to 18"?
I used to shoot doves, quail , and ducks with an old Mod 12 riot gun with a Cutts Compensator. Did a pretty good job. I currently own a 22" barrel for my A-5, with Invector chokes, and a 21" Mod 1100 Special field with RemChokes.
Briley's a good start. Just about any competent gunsmith can do the job.
