The 870 is an extremely common model...I would suggest calling your local dealer and having them look at it Or better yet, call Remington! They will probably be willing to send you a part for free that you can fix yourself
Most likely Remington will either want you to send it in or take it to a gunsmith. If you want to do it yourself you can check Brownell's or Numrich, who should have not only the part but a manual or schematic to help you.
If it is the ejector that is broken, then I would take it to a competent smith or send it to Remington. It is attached to the receiver needs a special tool to replace. The nice thing about 870s is that there are over 10 million of them out there and parts are relatively inexpensive and there are how-to videos and manuals around the internet. If it is the extractor that is broken (attached to the front of the bolt), then that is pretty easy to change out.
Its not an expensive fix, and they parts and tools can be bought online, That is if your handy. Word to the wise, Most smiths I know Charge more if the gun is brough in a shoebox and requires extra work by incompetant idiots. If you are mechanically inclined 99% of guns are very simple to work on. I've torn dowm most every gun ive owned and the only one I wont tear down is my Ruger 22 pistol.
