A quick check of the places that I normally look for Remington factory barrels yielded very little ..... new or used. If you're not in a hurry, you can do some occasional internet searching using the product code (F242641) and see what might pop up.
Here's two sources that have it in-stock, albeit not discounted:
I ordered an extra barrel for my Wingmaster 870 20 gauge LW directly from Remington. I called them up and placed the order over the phone. Pricey, but worth it. This might still be an option...