Check Gunbroker, I have bought a few "new condition" barrels on there for $50 to $100 for 870's and 11-87's with chokes. You will see a lot of them listed in the $200's but they never sell and just get relisted every week. This only works if your not in a hurry as it usually takes a few weeks for someone to list one at a decent price and no reserve.
The local gunshop here has barrels hanging like wind chimes from the ceiling. he usually gets $75 for a used barrel so you might want to check your local shops for a replacement too.
If your set on having chokes installed in your barrel that works good too but I just prefer off the shelf chokes. I had Tru-chokes installed in a 1300 that I shortened to 18.5" and they worked great but the chokes are an "order only" item, you can not just pick them up at the local Ganders or Bass pro. Carlsons would be my choice when I shorten the next one.
"A free people ought to be armed" George Washington 1790
‘For those who would give up essential liberty to purchase a little temporary safety, deserve neither Liberty nor safety.’ —Ben Franklin.”