Firewater is made by combining Gnomish Spirits and Water Flask in a Toolbox, all three of these items are sold by basically every Tinkering Supplies Vendor. This is one of the more common Tinkering subcombines in the game and is used in tons of different recipes. It has an extremely low trivial (17) so you'll almost never fail the combine even when you first start Tinkering.
Follow the links below to learn more about where to obtain these items. Almost every Tinkering Supplies vendor sells them.