In the UK, at least, a large part of the answer is zines: small, specialist, generally sporadic, low circulation - from a few dozen to a few thousand – magazines and journals, usually run by a single editor or a small group of like-minded obsessives. These might sometimes be found in specialist bookshops, especially radical ones such as Compendium, in central London, but for the most part they were something one stumbled across a reference to, and wrote off to an address for and subscribed to.

In the UK, Lobster, Robin Ramsay's journal of parapolitics, commenced publication in this form in 1983 and continues to appear today; it's one of a tiny number of consistent, persistent and referenced publications in this field. In a related field, Magonia (which ceased print publication some years ago, but is also now a presence online) cast an intelligent and acerbic eye on 'popular vision and belief,' not least UFO conspiracy theories.

There were sufficient publications of this and associated sorts (very varied, but all sharing the common thread of being 'intellectually radical') to form a Small Press Group, run by the 'radical traditionalist' pamphleteer John Nicholson and his partner Celia Boggis (of Private Eye) – pictured – from the late 1980s. SPG members swapped subscriptions and held an annual 'fair', and this was for some years prior to the advent of the internet one of the main ways in which links were forged between a very disparate group of publishers and publications.

One visitor to an early SPG event described an organisation that was 'firmly on the fringes of publishing which leads to both good and bad aspects':

Unsurprisingly, for a fringe operation, the fair attracted some pretty alternative people. Beards and bozos predominated, mixed in with Visa-accepting anarchists, poets, UFO loonies, children's authors, soft porn merchants (or perhaps they were art magazines) and simple enthusiasts. There were also three blatant transvestites walking around, presumably as a publicity stunt, but it is hard to keep an open mind on a hairy six footer in fishnets. This diversity of publishers is bound to throw up some weird writing topics and indeed it did. I spotted books on leather fetishism, the world domination of mysogynists, the works of Jane Austen, pub games, Tintin anarcho-parodies and ghost tours of Cambridge. You want variety? No problem. If I had the time to investigate I'm sure there is a lot there worth my time but for now I have to be selective and concentrate on what I know I'm interested in.

follow-up question: how far back can we trace conspiracy theories as we know them today?

Were there people in Ancient Egypt who believed the Pharaoh was really a Greek imposter or the like? What's the earliest known conspiracy theory?