In order to answer your question, we need to begin with a couple of important definitions. Colonization happens when a group of people move to a new place and refuse to accept or adopt the native culture of the new area while insisting on adhering to their own (from their own native place). An opportunist is a person who places selfish desires over morals or beliefs. It is pretty easy to see, even in disregarding Fanon’s book, that the two ideas go hand in hand. Through his book, Fanon shows that he supports decolonization because of the selfishness that accompanies the opportunism of colonization.
The first way Fanon shows opportunists assist in colonization is by exploitation of natural resources. Note Fanon's support of native peoples here:
For a colonized people the most essential value, because the most concrete, is first and foremost the land: the land which will bring them bread and, above all, dignity.
Fanon shows that the natural resources and the raw materials of an area are the first things to be misused. Opportunists, looking to further their own selfish desire, will note the abundance of certain natural resources (such as trees, fresh water, food sources, etc.) and take advantage of them. One can’t help but think of how the colonist of the American west began killing the buffalo for sport, leaving the carcasses to rot on the prairie. This is true exploitation.
The second way Fanon shows how colonization is supported by opportunism is through the existence of prejudice towards the native people of the place to be colonized.
Perhaps we haven't sufficiently demonstrated that colonialism is not satisfied merely with holding a people in its grip and emptying the native's brain of all form and content. By a kind of perverted logic, it turns to the past of the oppressed people, and distorts, disfigures, and destroys it.
The opportunists who attempt to colonize often refer to the native peoples as “savages,” such as the Eurpoean colonists referred to the Native Americans of what became the United States of America. This makes it easier for colonists to justify the violent treatment (focused upon in the next paragraph). This justification is important for further action against native peoples and the continuation of selfishness to occur.
The last way Fanon proves colonization would be supported by opportunists is by showing a further example of exploitation: inexpensive labor through force of the native people. This is truly “using” the natives of a place for an opportunist’s own gain.
The starving peasant, outside the class system is the first among the exploited to discover that only violence pays. For him there is no compromise, no possible coming to terms; colonization and decolonization is simply a question of relative strength.
Through violence, the colonists often force the natives of a particular place into their own service. Can there be a better example of selfishness? The important thing to realize here is that the natives, at some point, no longer give the assistance willingly; therefore, the colonists selfishly “take” the help from the natives by threatening or committing violence against them.
In conclusion, one can see that Fanon, as an avid psychiatrist and writer, believes that opportunism leads directly to colonization. It is important to take the entire book as a whole to discover this concept. As a result, I was forced to edit the more cryptic “answer with page number of bookstore” out of your question and answer with the most appropriate response. Quotations, of course, are even better than page numbers in regards to supporting this argument. For example, Fanon says, “Zombies, believe me, are more terrifying than colonists.”