A good source is eBay.
Gun Parts sell both used Remington and Mossberg barrels made for the 870.
You'll probably have to email for availability.
http://www.e-gunparts.com/productschem.asp?chrMasterModel=0860z870
Wherever you look, be SURE that you buy a Wingmaster or Police barrel NOT an Express barrel.
The Express barrels will likely NOT have the spring detent in the front face of the barrel support ring like the Wingmaster and Police guns do.
Take note that most sellers on eBay have no idea what you're talking about, so take care to be SURE you and the seller know exactly what's being sold.