You're actually better off just adding Xylene to the tank. Most of these octane boosters don't really add much octane, but they do foul your plugs with red deposits.
A gallon of Xylene in the tank is much more effective and it costs less too.
The only one that I've used that actually made a difference was Torco Accelerator...but it also leaves deposits on the plugs and O2 sensor.
(to answer the question...I don't know where you can get it)
John