I'm having the same problem... Petland in Kahala occasionally has silkworms available but my Jackson prefers crickets or mealworms. I want to start breeding land snails or roaches but don't really know where to start.
I would wild collect Schistocerca nitens. They are an invasive species that is a great large feeder insect and females could possibly be to big for a Jackson's Chameleon. However you can captive breed them similar to the way chameleon keepers captive breed dubias colonies on the main land.
Here's a quick Wikipedia link with some information about them on the Hawaiian Islands.