This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
Commercial fonts can be obtained from a number of different companies, including the large font houses: Adobe, Font Haus, Font Company, Bitstream, and Monotype. At these companies, fonts cost about $40 for a single face, and must be purchased in packages. Adobe, Bitstream, and Monotype also sell pre-designated type collections for slightly lower prices.
There are also a lot of PD reseller who have a vast quantity of fonts, check out your local Amiga magazin for more information.
Please consult the vendor list for a more complete list of vendors.