The Earthworm Pods, which differ from typical Earthworms devices, require 100 earthworms each. That’s the equivalent of 20 grams, and achieves the published optimal stocking density (150 earthworms per litre). 2 oz of earthworms equip 3 pods, and I think that this is the smallest quantity earthworm suppliers sell. Most sell worms by the lb or 1/2 lb. If you end up with excess worms, you can always place them in your garden, or try to resell them on craigslist. You’re welcome to include a link to earthworms for sale (and your location on the planet) in a comment below. Below is also a list of the suppliers I know of. Let me know in a comment if you know of others I should add. The smaller the quantities they sell, the better (but no fishing bait-prices please!).
List if earthworm suppliers:
Toronto: Cathy’s Crawlers
Massachusetts: Cape Cod Worm Farm