You can get loads of merchandise of Burnett's Vodka on the official website with the brand T-shirt being sold in an Extra Large on this page:
The Burnett's Vodka products they sell, range from party supplies such as shot glasses and coolers to BBQ grills, so everything you could need to stock up for the party season. Also if you want more clothing, there is an official Burnett's Vodka Baseball Cap for $12.99.
Unfortunately the site doesn't seem to have any other sizes of shirt, so one option is to follow Burnett's Vodka on Facebook, because they put on lots of competitions and give-aways, so you may be able to win one by keeping an eye out. Check out their Facebook page here:
If you're not keen on waiting around however, another way to get what you're after is to print yourself a shirt (though you might want to ask for permission first, just to make sure you aren't doing anything against their copyright). Check Spreadshirt for methods of cheaply designing your own: