As a foreigner, you will have no trouble. They never ask for ID or anything like that when you're buying booze. If they do say no, just seek out one of the beer and sake vending machines on the street or start going, "Eh, wakanai, eh, nani?" Unless the clerk knows English, they'll probably give up and just sell it to you. What do you mean by conventions? There is always something happening in Tokyo, here is where I go for info
Metropolis