Make sure spawn speed is set as fast as possible. If you have a Gourk to detect them, all the better, as well as Spirit Eye. Bad luck on one crab, kill it for a good amount of experience, then try to pickpocket the next. I also had really bad luck, even at rank 10. A few of them cooperated *eventually*. Basically had to sprint around the shore, having more luck along the western side.
If you don't want to do all this, it's possible to just duplicate the shells (or the breathers/masks, which I have admittedly done after so many playthroughs). Let me know if you want instructions to do that. They are pretty easy to find on the Ironworks Board, as well.