Rather than buying new parts, I would remove your current plunger, clean it and it's recess, then lightly oil and test. If the plunger and recess are already clean (or cleaning them does not help), you can cut the plunger spring. Its a headache, because you have to re-assemble the lever to try it, but cut a little at a time (1/2 coil). I wouldn't bet on a new spring curing it. You can also polish the plunger where it rides to lock the lever closed. Last thing to try is re-shaping the plunger so it has less "hold", if you get carried away, your lever will pop open when the magazine is loaded (I have a 336 which does this).
I filed the point off a small nail for a punch to remove the plunger pin. Use a longer nail than I did, there was a lot of thumb-smashing when I fixed my 1895 plunger.
Andy