try looking up shirts from threadless, they have lots of trendy t-shirts made by freelance artists shirtwoot is also a good place for t-shirts as they're pretty selective about what they feature so you know it's got to be good stuff (:

Personally, I love Claire Belton's t-shirt designs as they're simple and very cute her shop (she is QueenofDorks on deviantart)

DesignByHumans is an awesome site that people post their designs and if they get enough votes the site sells them~ so you could get a good idea of what is popular there~!:)