Perhaps the first kami reference was in the early 8^th century when the Kojiki text spoke of kami inhabitance. This literature tells of kami in the sakaki tree and represents the ancient thought that kami can dwell in tree hollows, gourds, and other small spaces. Thus, the concept of omamori has been a part of Japanese culture for many hundreds of years.

http://www.greenshinto.com/wp/2011/12/12/sakaki/

One of the most popular types of omamori, hyōtan, originated in the Tokugawa period and are much the same today as they were then. However, concerns are changing, and the popular prayers are evolving with society. Before 1950, most omamori were made of wood and paper, but today they are made of plastic. They used to be made by lay women; today they are mass produced by factories. With the technological advances made in the past century, omamoris have made their way online where they can be purchased without worshippers even needing to visit a shrine. The internet uses catalogs targeted to priests to sell amulets wholesale and to publicize the ever-growing number of talisman and amulet variants available. The Izutzu catalog has 90 pages devoted to more than 275 versions of omamori. From bumper stickers, key chains, pencils, credit cards and plaques to demon-smashing arrows, foam finger rings, carvings, bells, porcelain, and pieces of paper, amulets come in many forms and are available to many individuals.