At one time finding barrels for 870's was relatively easy and inexpensive enough to be worth the effort. But you may find it just as easy to buy another gun today. You're more likely to find a deal on a longer barrel then cut it shorter. I've done several. If you cut right on the vent rib post you can choose 21", or 19". Any shorter and you get 17" which isn't legal. I used a front sight that just clamps onto the rib. Virtually no cost other than the barrel.
There is a date code on the barrel. If the barrel is the same one that came on the gun from the factory you can look up the date it was made here. There is a 2 letter code on the left side of the barrel near the receiver. For example AK would be March 1963 according to this chart.
Remington Manufacture Dates (oldguns.net)