Sometimes the best thing to do for the animals at those stores is to just move on. I never buy animals from stores that don't take care of them, it just rewarded the stores for mistreating them. I have a theory that petco takes bad care of their animals so people feel bad for them and buy them.
Does the silkworm cake save the juices in the worm or does it dry them out? There is nothing better then biting into one of them and having it squirt in your mouth.
Look at the site sponsor list, it is full of good places to order. I have ordered hornworms from Great lake hornworms plenty of times and I am always happy with them.
I would stay away from butterflies as there are many poisonous ones. I do feed moths once and a while when I catch one. I would also stay away from meal worms as well, hard to digest and little nutrients.