Well, you could buy a pint bottle of your alcohol of choice and take that, use a hip flask, or just take the plastic bottle (remove the label).

I'm going to an 18th of an old friend of mine and it's byo. I don't have a huge supply of alcohol and at parties i usually go it it's acceptable to bring a plastic water bottle with a little vodka (or similar) in it & mix it there. I feel like this might seem a bit tacky...so Im looking for a slightly more classy way to bring (& drink it) at the party.