Look to see if there's a horse farm or stable near you, or even just a regular farm. They often have piles of horse manure or compost sitting around, and they are most likely jam packed with worms.

I just typed my city and "stables" onto google and got a whole list of results, gave some a call and asked them if they had piles of aging manure sitting around. If they said yes and allowed me to visit, I gathered my gear and at the end of the day about a lb of redworms were mine!