I got mine on Ebay and AuctionArms....you may have to wait a while before what you're looking for turns up, but the winning bid is usually around 50-60 bucks. The best success I've had is to do a search on Ebay, checking the Sporting Goods auction, and doing separate searches with the following terms:"870 extension," "870 extention," and "870 mag ext." Sometimes folks will misspell their auction item.
Most common is the extension for 18" barrels, those for 20" are a litle harder to find.
Good luck