Any metal supply company will have it. There's nothing magical about "billet" Aluminum, it simply means wrought Aluminum stock. This means it was formed into its shape (bar, plate, round, et.al.) by some process (rolling, extruding) other than casting and has better properties. All machining stock (bar, plate, round) is billet material. Sometimes aftermarket suppliers will brag about using 6061-T6 billet aluminum. This is the most common off-the-shelf aluminum machining stock available, i.e. it's not special. The numeric designation describes the alloying composition and the T6 is the heat treat designation.