http://www.greenlinnet.com/

http://www.rounder.com/

http://www.fellside.com/

There were some more I was thinking of, but they seem to have gone under.
A Thistle and Shamrock is a good place to hear celtic type folk music. Another NPR show is The Folk Sampler, with more Appalachian type and other non-Celtic music. I have also heard Celtic Connections and Our Front Porch, which are available on some NPR stations.

I know a lot of sites with lyrics, but not so many with recordings for sale. I usually look up individual artists and order from whatever they have available. Some of these people are pretty obscure and can't be found on most record sales inventory.
I sugest going to a few folk/bluegrass/celtic festivals and get an idea of what's happening in this type of music to get some ideas. Besides, the live acoustic jams are great!

Dave