The packets are a big rip-off.
The activator is yeast, you can buy bread yeast at the grocery store for Much Cheaper. Amount to use each time, 1/2 teaspoon.
I've heard some people use wine yeast, supposedly lasts longer and is more consistent with co2 production. Probably have to order that online, unless you have a home wine-making shop in your area.
The stabilizer is baking soda, which you likely have in the house already. Obviously also available at the grocery store. Amount to use each time, 1/2 teaspoon.
Somebody else please chime in here, I'm not 100% sure on the 1/2 teaspoon thing. (I switched over to all pressurized co2 about a year ago now.)