The yeast that I use is a yellow, dry, flaky substance grown on molasses for the specific use as a nutritional supplement. It's different from brewer's yeast which looks similar, but tastes much nastier. I think it may also be called torula yeast and is sometimes sold with the brand name "Red Star". Red Star also makes a brand of baking (bread) yeast which should not be confused with what I sprinkle on popcorn.
I think, though am not certain, that Vegemite and Marmite (popular sandwich spreads in Australia) are a semi-liquid, paste-like form of the same product.
I buy this yeast at natural food stores that have an extensive bulk foods (sadly, a rare thing here in the NY-Metro area) departments and purchase it by the pound. If you go to one of those natural or health food stores that are really vitamin stores, you are unlikely to find it. Try a natural food co-op, Whole Foods, Wild Oats or other true natural "FOOD" store that emphasizes the word "food" in its approach.
If you find it, check out the nutritional makeup. If there is a more perfect food, I challenge you to find it. High in protein, low in carbohydrates, no fat to speak of (maybe 1 g/2 tablespoons yeast) and loaded with B-complex vitamins.
Don't forget--it tastes really good. Kind of like parmesan cheese.