You can buy duty free items in the airport. One word of advice I have about buying beer at the duty free... make sure your luggage doesn't go over the maximum weight allowed by the airline. We sat next to a guy on a recent flight that bought 2 cases of beer because they had a good deal (2 cases for $20 I think), but when he checked his luggage, he was a few pounds over the maximum and they charged him $80.00 US extra. That made those cases pretty expensive.
If you are going to carry it on, you shouldn't have too much trouble, but it might be easier to just stop at a grocery store on the way from the airport... it's not that expensive to buy there and it saves having to carry it all over the airports. Most towncar companies allow a free grocery stop or if you're renting a car, you can easily make a stop.
Good luck whatever you decide to do!