You can search online by clicking the several key words to find which type of sunglasses that Johnny Knoxville wears. Then you can base on this type and find the similar one online. Many online stores must have this type because the stars make it popular. And buying online is cheap which will save a lot of money for you.
I just search it in Google Image, most of sunglasses that johnny knoxville wear is aviator style and wayfarer style. But i don't know which brands he wear. If you want to make yourself look like him, you can choose a pair of similar one or the same style sunglasses. And aviator style and wayfarer style sunglasses can be found everywhere.