A few whiskey distilleries sell 5-15 gallon barrels directly off their website (Woodinville, Corsair). Some of the homebrew shops sell whiskey barrels from distilleries including Balcones (TX), Ranger Creek (TX), Dad's Hat (PA). I believe Farmhouse Brewing Supply gets various other types of barrels and advertises them on HBT. A lot of craft distilleries with these smaller barrels will sell direct if you contact them.
Tequila barrels are going to be tough to find. Tequila is only made in Mexico and mostly in full size barrels. Tequila is usually aged in used bourbon/whiskey barrels so you could always pick up a whiskey barrel, run a beer through it, add some tequila to it and get pretty close to an actual tequila barrel.