You can always go out and find them. Look in your compost, if you dont have compost look in a friends. Parks often have areas where they dump organic material too. They will generally be in large groups in things like plant stocks, old sod, composting cardboard, paper, and ect. Manure piles are also a good source. Just a suggestion but I know some people don't want to go digging through compost.

I've never tried this myself but I've read that it works. Take a large flat piece of cardboard (like a box that's been cut down) and dampen one side slightly. Then lay out on your lawn or an area of dirt in your yard in the evening. In the morning, go check under the cardboard for worms. This might take a while to collect any significant number but it's a good start. If the ground or grass is already damp or wet, you won't need to wet the cardboard first.

Finally, if nobody responds here, you can try freecycle.com, craigslist, and/or vermicomposters.com . I think people want help others start vermicomposting but they also want to be sure the worms that they give won't be wasted. So, to make it easier for people to choose you to give to, try posting a description of your worm bin and where you'll be keeping them. Let them know you've done your research and take this seriously.

Good luck.

I discovered vermicomposters.com from another posting and found someone local. Thanks for the responses