A good NOS alternative that is readily available and requires no vehicle modification is VP100. This is a product that is composed of 100 octane, unleaded gasoline that is oxygenated with ethanol (alcohol). It is environmentally friendly and is legal throughout the United States. It is also considered a CARB legal fuel, so there is no problem using it in California. It is safe for all catalytic converters, and will not harm oxygen sensors. It claims to produce 14 percent more horsepower than vehicles using straight 91-octane gas, and will work with cast-iron engine heads, aluminum heads and computer controlled turbo engines.
For more info, see How Nitrous Oxide Increases a Vehicle’s Performance