I understand that just about all of underground commerce was (and mostly continues to be) word of mouth, but I've always wondered how it worked with electronic devices where a large amount of resources is required to create a product.
I'm a little fuzzy on sources here, since I haven't read much on this topic since the late 90s, but from my understanding, it would indeed be by word of mouth, with some important aids: BBSes, cryptic (and risky) classified ads in magazines like 2600, and contacts at specialty electronics stores. A lot of this stuff, however, wasn't sold as a fully functional piece of equipment--instead, you would buy a perfectly legal set of blueprints from a vendor in 2600 or wherever, and it would include a list of sources for the various components. You would then buy a board and solder everything together.