Walmart, there should be a fishing/bait section and a fridge where they have canadian nightcrawlers, red wrigglers, and some times leeches.

They are $3.25 for 18 where I live, should be about that price for you too. I have fed these worms to my clawed frogs for almost two years and can report no problems.

Any bait shop worms will do, just make sure they don't contain dyes or anything odd to 'enhance' them to be better bait. You just want plain old earthworms.