You can buy it from Roavar the bartender for 5 gp. See the cooking guide for information about brewing this using mushrooms as the main ingredient.
Player-brewed ale can be drawn out of the barrel into mugs or into calquat kegs which hold up to 4 doses. Ale can be poured out of kegs into mugs, but mugs cannot be poured into kegs. Each dose of ale from a keg is the same as a mug, with the same effects.