You can order it off amazon or go to whole foods. There's both pill form and powder. Just make sure it's Brewers yeast not nutrional yeast. It's not the same thing. Personally, I prefer the Solgar brand. There's no weird after taste whenever I add it to my oatmeal or use it for baking.
look up home brew stores in your area. They come both liquid form in tubes and dry form in packages. I would imagine you would use the dry yeast since you're cooking. There are lots of strains but just ask the store clerk for help.
Re: Brewers yeast