I finally managed to remove the tub spout thanks to some tips I found here, and the leak is still happening out of the bare pipe.
I've got an older (late 80s) shower/tub combo with single handle - probably Moen. There is no plumbing access space on this one, just a wall, open to the room on the opposite side.
Not sure what my next step is - I am thinking it is time to cut into the wall to access the actual valves and stuff. Any advice? If it helps, the house is for sale, so let me know if I should just leave the leak and take something off the price.
It's held in by a horseshoe-type clip and it can be difficult to remove sometimes without the cartridge tool, but you should be able to get it out, take it to the store and get a duplicate. It can't fall behind the wall normally. Just make sure that you put the new cartridge in the exact same way that the old one was seated.