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!