I can't comment on cockle availability, but I can say that keeping a puffer's teeth trimmed is a near non-issue in marine tanks. Contrary to popular opinion (curiously, this is made up almost entirely of people who don't keep puffers), they do not attack hermits and shrimp for the sole purpose of wearing down their teeth. I have actually heard this as the main argument as to why they are not "reef safe". Rather, the feeder can feed them "regular" foods and the puffer will actually bite rocks and coralline algae. Whether it sees an amphipod or is actually self-aware, I cannot say, but I have owned several triggers and puffers and all could be seen biting rocks from time to time. Mine today even avoid corals doing this; indeed, so far my "un-reefsafe" fish have been 100% trouble free.