It's not that they're "underground", it's that they don't want to kill their reputation for quality by offering their products in the worst audio stores known to man. They are sold in brick and mortar shops, but only those that actually know what they're talking about, which is definitely not going to be tweeter, car toys, etc.
This has got links to most of the manufacturers out there, but I'm sure I forgot a few:
http://www.resnet.trinity.edu/areynol1/audiolinks.htm