Antimatter is rarely found in nature, and always very thinly spread in the vacuum of space as a byproduct of high-energy processes. The harvesting of such space-borne amat is a specialised task, usually carried out in the magnetic field of large gas giants or of stars. More details on Antimatter harvesting here.
Alternately antimatter can be manufactured in an Amat farm, a field of orbiting stellar power collectors which feed massive orbiting particle accelerators for the production of bulk antimatter.
Antimatter is also difficult and extraordinarily dangerous to store. The only reason early modosophonts were compelled to endure the extreme difficulties with it at all was the truly incredible performance it offered compared to fusion, and the fact that antimatter drives are the first practical interstellar drives. An antimatter drive is always used to convert matter directly to energy, and then that energy is used either as direct reaction mass, or to heat another reaction mass.
Despite the theoretical foundations being worked out as early as the Information Age, the difficulties of dealing with antimatter - including producing and storing it - meant that commercial mass-production and use had to wait for several centuries. With the development on the one hand of safe and practical storage devices (using powerful electro-magnetic containment rings and multisensor feedback), such as the Pionomak and the Plasma Helix, on the one hand, and the rise of the Amat cartels like Prometheus Inc, Nova Industries, and Helios Corporation, with their gigantic space-based solar stations which used lasers and electron beams to create particle-anti-particle pairs in industrial quantities, anti-matter finally became a practical means of propulsion.
Even so the cartels kept prices high, amassing vast wealth and political clout (not unlike the Petroleum producing nations of the late Industrial age) and going into partnership with the big nanotech companies like Neotek, NanoIndustries and Adaptive Nanosystems.
Even in the Current Era many planetary systems have at least one amat farm or amat collection array, although Conversion technology has largely replaced antimatter technology.
Transapient Conversion technology can be used as an energy source if available. More on Godtech amat production here. But difficulties maintaining and using transapient-tech, and the fact that these devices are rarely given out to lower toposophics, means that even today most amat production is via vast starlight-powered amat farms, or gathered from the magnetic fields of great gas giant worlds.