One of the best places to find independently designed tees is on the internet! There are literally TONS of sites that offer strange and unusual t shirts for your indie fashion, and man is here to help you weed out the truly indie from the commercialized garbage!
Etsy is a great site for finding ironic t shirts! Folks from all around the world post their handmade apparel (as well as many other products!) on Etsy, so everything is unique! You won't face the humiliation of seeing a fellow hipster wearing your incredibly unique t shirt if you shop at Etsy! Clothing here is a bit expensive, but when you purchase handmade you have to pay a premium!
Woot offers cheap, unusual shirts with a different one posted every day! With free shipping and independently designed shirts by their users, Woot is a always a good choice for your indie tees!
Good luck, and happy tshirt hunting!