i know this isn't much help to you, but, if you're going to a flask it's worth your while to get a quality one. those thin walled, chinese made ones you can get engraved at the mall are crummy and not worth your while.
i used to have a really nice, solid flask from pottery barn many years ago. it was stolen from me in a robbery, and of all the stuff i lost, that was the only thing i really missed. i can't find that flask anymore at pottery barn
but look for nice, thick walls, and a good cap with quality threads, and (like above), rubber gaskets are nice. it's nice to have a cap that is attached to the flask so you won't lose it, although it looks nicer to be sans retention system.
i'm not a flask "user" anymore, because i've never been able to find a flask as good as the one i used to have. i was tempted to buy a titanium flask, i've seen posted on these forums a few times. it was like 100 bucks or so, which is kind of steep, but the allure of titanium makes it kind of nifty.
oh, and make sure you get a tiny funnel to go with your flask. don't forget this or you'll waste a lot of booze trying to fill your container.
best of luck!