The three places you will find Native American inspired merchandise around the nation are:
Traveling Vendors such as Flea Markets and Concessions (extremely common around military posts, which also attract purse vendors).
Native American Pow Wow's (seasonal gatherings of people from the culture, generally open to the public,
www.powwows.com should list large ones).
Near/On a Reservation or Casino outside of Vegas. (I haven't seen that stuff at every casino however).
If you're in the southwest, there are bountiful country western themed shops that sell leather goods of both the frontier and native origins.
I would personally avoid the internet, as it's probably junk made in a sweatshop, mass produced, and Native Americans make FAR superior leathers.