Here’s my source for blackworms: https://shop.easternaquatics.com/product.sc?productId=1&categoryId=1
Owner’s name is Craig Shaubach, and he’s very responsive via email.
I had some difficult fish once, and thought I’d place a one-time order for blackworms. They were such a hit in my fish room (even ancistrus plecos loved them) that I’ve made repeated orders ever since!
They’re small, so they’re not a very efficient way to feed large fish, but they seem to break the spell for stubborn eaters. And as a bonus, they are aquatic, so they’ll live indefinitely in your aquarium.