How much yeast to add to soda per 1 gallon?

[SodaMaker55]

How much yeast to put in soda per gallon? I am new to making anything carbonated, and all I can find are recipies for kegs. I want to do natural carbonation l. Just a measurement of yeast per 1 gallon of any soda.

Also, what kind of yeast should I use? I am thinking champagne yeast.

Where can I find champagne yeast? All my brew shops are closed and I dont feel like paying 8 dollars for shipping. Or getting 15 pack at a time from amazon.

Thanks!