I've heard of people finding red wriggler worms at walmart I certainly can't vouch for them. I ordered mine years ago from an online retailer of worms and they've done well for me. They arrived in a little bag and I was uncertain if they'd make it, but they did!
As for the castings, I use a five gallon bucket full of water, into which I put a couple "double handfuls" of castings. This I mix up along with a bit of molasses and then I put an aquarium bubbler into it and let it bubble for a day. The next day I use it.
