Ken Westerfield was endorsed by Adidas Canada (1974-1979), but, he mostly wore existing Adidas footwear and apparel, rather than sportswear designed specifically for the sport of frisbee. That said, Adidas did release their own frisbee, which, according to my research was manufactured by 'WHAM O' for Adidas; Wham-O Inc. is an American toy company - it trademarked the term frisbee and pioneered the toy - that is located in Carson, California.
Frisbee became a popular sport in the 1960's and 1970's, in part, due to the adoption of the toy by the hippie counter culture - mostly located on the West Coast of America - that rejected the physicality and aggressive nature of the traditional American sports. There are three main types of frisbee competition: freestyle, ultimate and disc golf. Ken Westerfield became a champion in each style, and became a figurehead for the sport in the 1970's.
The popularity of frisbee as an alternative sport began to wane in the 1980's; which perhaps, was not surprising, as it coincided with the decline of the hippie movement and the rise of the materialistic driven 1980's 'yuppie' culture.