Worms?

I would like to introduce more worms to my garden and hopefully my entire yard. Does anyone know of a place that ships worms? Or should I just go get some from the local bait and tackle? My yards very hard clay, I've tilled peat and manure into my garden plot and it's a lot nicer now. When I tilled that area in the fall, I didn't find a single worm, so I'm hoping to improve the situation.