To avoid pit-stained monotony, your T-shirt inventory should be regularly updated -- but endlessly trolling vintage shops/your friends' dressers defies the carefree attitude that tees are meant to convey. So, subscribe to T-post.
TP is a Swedish company that creates new designs every six weeks, then ships them to your door -- like your magazine subscriptions, except not wrapped in black, smut-obscuring cellophane. The designs are based on current events, some of obvious import ("Shotgun Golf with Bill Murray", commemorating Hunter S. Thompson's final article), others more obscure ("I Will Swap My Shirt for This Creature", inspired by that bastard Canuck who internet-parleyed a paperclip into the house your toner cartridge should have secured). Quality-wise, TP prints onto American Apparel shirts -- because while Swedish socialism's accomplished universal healthcare, it's apparently incapable of sewing cotton.
You can scan "back issues" in the site's Gallery, but you can't buy them, as TP only creates as many shirts as it has subscribers. Sign up immediately, and you can go back to using your buddies' clothes in conjunction with your magazines.