I used a letterpress printer in my local area. Yes you are going to pay out of pocket, and have to distribute/sell them yourself, but in my experience, all went well. Also it's good to build face-to-face relationships and keep the printers in business!
I always thought the "underground" publishers meant the DIY or at least the DIY-type movement in terms of what dmilam has suggested. Not only do you keep small printers in business, but after a while, you get to know the network of other people that are doing the same thing (now that there's the Internet!).
Your local letterpress may very well be a small press and you should always hit them up first. You can also check on PW.org for independent publishers or small presses, which I'm sure is the easy answer you came here for. I only post it for most everyone else wondering -- the ones who don't know about that directory. Though, most people that have done their own homework know about it already.