Check your local pawn shops. Right now there are at least four 870s w rifle bbls in my local shop. Price is about $100 less than new.
Good luck.
I wouldn't go to a pawn shop.
http://gunbroker.com/is wonderful. If you purchase from this site, be sure you purchase from a seller who has MUCH positive feedback. I bought a Mossberg shotgun &love it. The transfer FFL was surprised that I only paid $125.00 for it and said it's about 10-yr old but looks like it wasn't really used.