I would first confirm that your shotgun needs it. Set up a stoppage with a dummy round between the lifter and the bolt in the forward position. Feed it into the magazine tube, push on the shell latch to release it, and allow the lifter to drop down.
If you can't clear it just by pulling the forend back hard and you need to pull out the trigger group, the Flexitab parts will fix it. Some older 870s will get tied up this way; others will not. Mine is one of the ones that can be cleared just by working the action.
I would try Western Gun Parts for the kit. Brownells has them, but additional export paperwork may be required.