I produce my own. Its easy. Below is a recipe I found on-line. I have tried it but half of the cultures molded up and died. What the recipe needs is a mold inhibitor. Josh's Frogs sells this as well as their own fruit fly medium, which I swear by.

Harvey Peterson Medium

1/2 cup Warm Water
1/2 tbs. White Sugar
1 tbs. Powdered Milk
4-6 tbs. Instant Mashed Potatoes
5-15 granules Baker’s Yeast

In a 24 or 32 oz. deli container or insect cup, dissolve sugar in warm water. Add powdered milk and stir well. Add mashed potatoes (the exact amount required will vary with the humidity level and ventilation of the cultures). Swirl the container in your hands to thoroughly mix ingredients, then let sit for a few minutes to solidify. Add yeast on top, then introduce fruit flies.
Alternatively, mix all dry ingredients together, then make up culture medium using equal amounts of mix and water