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What particle sizes? Micrometer or sub-micrometer/nanometer?
What kind of material? organic or inorganic, metals, glasses, fibers both natural and man-made... there are many possibilities and many ways to produce desired shapes.
Next question is: is this just academic interest, where small gram or even sub-gram amounts can be employed since cost and complexity don't really matter much, or are you interested in methods for bulk amounts that are used in the real world?
There are two major routes, 1. bottom-up where you "build" structures from smaller components and 2. top-down, where you have larger building blocks which are then "destroyed" producing smaller products.
It is usually best to start with existing materials that might occur in nature or are easily commercially available. These materials are then modified to produce the desired properties and shapes.
Example: for elongated fillers, start with fibers and mill them to the desired size. If platelets are wanted, certain minerals can be employed. Perfectly round spheres can be produced either in solution or by cooling a vapor so it condenses producing round droplets.
Nanometer sized materials are not often used, due to cost of manufacture. The number of real world examples is pretty small. There are many academic papers out there describing methods to produce all kinds of shapes, like hollow cubes, nanowires with pentagonal cross section, triangular nanoplates and even weirder shapes, but those aren't produced commercially (yet). These are often grown in solution or in the gas phase under carefully controlled conditions, sometimes by using templates.
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