In the hunt for earthworms, you can also try looking into home composting setups. Red Wrigglers,
Eisenia fetida, are commonly used for home composters. These worms do have foul tasting secretions though, some frogs won't go near them and some people claim trying red wrigglers put there frogs off worms completely (I've not had this experience though). Some frogs won't care and gobble them up happily like they're candy.
Whatever you believe about impaction, this pretty much covers my mealworm usage. I've kept a shoe box sized colony going for years with very little maintenance as a backup and also a bit of variety (always good

).