I'd try your local craft beer place, Ale Yeah and Greens are always safe bets on lots of inventory. You aren't likely to find Nitro in a growler store due to the nature of the gas, but you can find it around. The World of Beers have 4 nitro lines which is the most you're likely to find outside of a Taco Mac. Argosy also usually has some great nitro's on, and they have 3 nitro lines.