I'm building a t-shirt website and I want to be all legal and stuff. I want to offer the customers interesting designs, like the ones from threadless.com or zazzle.com.
So the question would be... where and how do I buy designs that I can later sell on my website? I know there are interesting sites like designcrowd where you can get nice designs for your ideas, but what I really want is existing designs that I can sell on my website.
There are a lot of places you can either hire designers or approach them for existing pieces. Hiring them would be more flexible on the legal side for you to use/reproduce the designs however.
For hiring, try freelance sites such as http://www.microlancer.com/, or even look into portfolio sites like http://www.behance.net/ or http://dribbble.com/ to approach someone with the design style you are after.