Where is guild vendors?

Excuse me, but could you possibly clarify for me the location of the guild vendors? I've been searching around for quite some time now, but I seem to be unable to find them. I've heard that they offer unique items and services that are crucial for my progress in this game, but I'm at a loss as to where they might be. I've checked the marketplaces, the main streets, even some of the more obscure corners, but no sign of them. Could you possibly point me in the right direction? It would be greatly appreciated.