Spook is processed from the vestigial remains of the oldest kind of synthdiet found in Necromunda. The decayed synthdiet deposits are now nothing more than a lurid green powder, having been acted on by mutant fungi for thousands of years. It contained a high proportion of recycled human protein and it is this human essence which is likely to account for its dramatic effects on the human psyche.
The drug spook is taken in liquid form - the ultimate magic potion. When drunk in small amounts, it awakens the imbiber's psychic abilities. When drunk in quantity it opens the channel between a person's physical body and their soul in the warp. If the individual has strong soul, it will be drawn into his material body; if he has a weak soul, all psychic energy will be instantly sucked out of him and lost in the void. It is for this reason that spook is a very dangerous substance, and its use viciously repressed by the Imperium.
In hive world society, people are constantly seeking ways to exploit anything they discover. The people who stumbled on the unusual green deposits investigated ways of turning them into wealth, as they would have done with any substance, and in the process discovered spook. Being ignorant of matters of the human soul and the danger inherent in mankind's metamorphosis into a psychic race, spook was seen as just another substance to be recycled and exploited for profit.
There has always been a massive demand for drugs in hive society, mainly to supplement the diet and ward off sickness. Spook became popular among the nobility who reveled in its exotic effects and it has slowly filtered down throughout hive society.
The noble households which exploited this resource naturally kept the trade secret and continued to grow rich. The household of the Lord of Necromunda himself was involved in the business and was able to organise off planet trade of spook. This had to be accomplished using smugglers, since the Imperial fleet conduct all legal trade in space.
No one knows or can predict where the spook deposits are to be found, but whenever one comes to light, the 'right people' are informed, and mining and processing can begin. Trusted noble households with a close connection to the ruling dynasty will get concession to exploit the deposit.
Small quantities of spook are also found and traded by scavvies who stumble on eroded deposits during their delvings. This accounts for a small amount of wild spook that is traded in the undercity and shanties. Imperial agents trying to track the spook to its source usually end up following the scavvy spook and thereby miss the main source. There is nothing that connects the nobility to this scavvy spook.
The most significant outlet for spook is the secret cults that lurk in many hives. These cultists need a regular supply of this psychic-enhancing substance. The Immortals in particular require vast quantities for their rites and the expansion of the this cult is certainly the single greatest factor in the growth of the spook trade. Most of the spook lords who rule the Forbidden Cities are probably already members of this cult.
Spook is easily distributed via the various undercity, scavvy or nomad gangs who ask no questions and only know of the next link in the chain.