A lot of gardening centers will have it, it comes in a small bottle and a large one, believe me you want to get the large, for $5 you quadruple the amount you have and you have to SOAK the plant with it to get it to work. Kinetin doesn't absorb easily so you need a lot, but it's worth it!
Guys if your trying to increase the lateral branching on your plants topping is a good method but if you want to avoid the stress associated you can buy Bonide Tomato Set Spray, the kinetin causes the branches to explode from the main stem!