Some Petland discount stores stock blackworms.
For me training butterflies, I have about a 50 to 25% success rate depending on how I calculate. Of the 13 butterflies I caught this past season, (excluding the ones that died for other reasons) I got 7 to eat live mussels. The rest died due to fighting and other general competition. If I separated them into different tanks based on size, I probably would have another 3 or 4 more today.
Go to the seafood market, buy a couple of live clams, mussels and oysters. Make sure they are fresh and not open in a bucket of ice, since the fresh water shocks and kills them. Toss one of each into the tank and keep the extra in the fridge. If the shellfish is still strong, it will live indefinitely in your tank and the butterfly will learn to pick on its mantle. It will eventually weaken it and then the fish will dig in to its meat. If the shellfish is dying, then as it opens up, the butterfly may take it, but if it dies too fast, dump it or chop it up for the rest of your fish to eat.
Then get the spare shellfish out of the fridge and do it again. It will take them, it's just a matter of time, and theres lots of pollution from the uneaten food.