One of the ways that I obtain material that is not commercially available is to find seed of rare species. Growing from seed is not the fastest or easiest way to obtain bonsai but it does allow one to exert complete control over the development of a bonsai.
While there are many ways to sprout seed I am currently sprouting my fig seed by sowing the seed on the surface of rock wool plugs. These plugs are available online or from garden centers.
The plug is kept in a shallow dish with a bit of water in the dish. This keeps the plug from drying out. Ficus seed take somewhere from 1-12 weeks to sprout or even longer. Plants are kept growing on this plug until they have 4-5 leaves on each sprout.
Once the seed has sprouted and two or three leaves have appeared the plugs can be placed into a bonsai soil mix. The surface of the plug should be 1/4-1/2 inch above the surrounding soil mix and allowed to grow for a year. Once the seedlings are quite sturdy with about 8 leaves on each one they can easily be removed from the plug and rooted into a pot of bonsai soil.
It normally takes 3-4 years from seed to get a plant large enough to begin bonsai training, so the process is not for the impatient bonsai grower.
Click below to read more details about seed growing.
The definitive reference work on Ficus
for bonsai. The book is a softcover, 8 by 10 inch volume, with 144 color pages, containing detailed information for the beginner as well as the advanced hobbyist.