I have an older 870 Express with the factory installed 2 shot magazine extension. I read the thread about the difference between the 870 Express and 870 P. I thought it would be fun to make the changes to my Express and get it close to the P. I have located the magazine spring and sear spring on Brownell’s website. However, I cannot find the machined extractor or ejector on there. Does anybody know where to find those two items? Thanks.
Have you tried to call Remington ???call them and ask if they sell the parts you need to you and if not ,ask them if they could give you a companies name that sells Remington parts for the 870.
Remongton is a PITA about selling 870P parts unless you are LE. The parts list can be found on their web site and some/many/most of the 870P parts are restricted. But I thought I saw what the OP was asking for at either Brownell’s or Midway.
When we ordered barrels and parts from the LE parts list last year they didn’t even ask. 
Not all 870P parts have the restriced code. Edited to add: They also could have loosened up on thier policy as it was pretty stupid.
I have found that you can find nearly all of the 870P parts on gunbroker.com