So you want to be an online retail store that sells different brands? I think you could find lots of people here that would probably let you sell their shirts on your site!
I don't know if you've heard of Dr. Jay's. They have physical stores in New York and New Jersey, but also an online store. They sell just about every "big" brand of urban wear and some that are a little smaller. Check out their site
www.drjays.com. Is this the business model you are shooting for?
Again, I don't think you would have much of a problem with smaller companies. But you need to keep in mind, that they would be selling to you wholesale. Which would mean that you would either have to purchase enough (a lot) to make it worth their while to give you a lower price than they retail for, or provide a site that gets lots of traffic and has high visibility (lots of marketing money!)