I am a large-ish apparel and textile manufacturer in India, and do the majority of my export to the United States. I am pretty well-versed with the kind of market you want to get into. My email is in my profile--get in touch with me and we'll talk. I doubt that I personally can supply you with what you need--we operate on volume--but I know plenty of people who can get you what you need.

A few things to keep in mind, though. I have just typed these in no particular order off the top of my head. I'm going to shy away from offering marketing advice and stick with what I know best.

(1) This is probably the worst time in history to enter this market. Cotton yarn is at all-time highs (70-80% increase over this time last year), and it's hard to get deliveries even if you can pay. The average Joe has no idea how many people between the cotton farmers and the store are struggling to maintain the shelf price of a garment. A great example of this is the fact that everybody in this thread is suggesting that you get your t-shirts from China. This is bad advice--China has increased labor costs and has tremendous yarn shortages for cotton.

(2) The small-volume t-shirt business is all about logistics. Getting t-shirts made and printed is extremely easy (getting them made cheaply is a little more involved). Arranging for the logistics of sizes, colors, styles etc. is complex and expensive if you don't have access to good storage.

(3) Please, please, please don't use CafePress/Zazzle etc. The quality is terrible and their prices are pretty outrageous.

(4) I take it you have very little experience in the industry. I suggest you spend some time doing your homework. I wouldn't worry too much about the chemistry behind dye science, but you should really be familiar with some basic knitted structures, your yarn options, dyeing options and you should have a solid knowledge of your printing options. Look at value-addition treatments to garments.

(5) Get a firm understanding of getting your t-shirts manufactured. If you are looking for poorer quality (but better price), Central America is a good bet. They use cheap dyeing methods and open-ended yarn, but because of some political help, they have amazing prices for USA importers. If you are happy to pay more, then import from Asia. You will not get duty-free garments (think countries like Egypt, Jordan or Kenya if you want duty-free), but you can get excellent quality. You also need to understand the duty structure of importing apparel into the United States.

(6) Most people who export to the United States quote prices for putting the container-packed garments onto the ship. You are then responsible for shipping, insurance, duty and trucking from the port to your DC. Trucking is painfully expensive. Take lead times from various countries into account. I also highly doubt you will be importing even a 20' container's worth of t-shirts in one go, so you need to investigate the route of partial container shipments.

I hope I didn't put you off--every industry has challenges. This one is still really fun and exciting, especially if you are new to it. There's a lot to learn.

[edit] I should add: I'd be glad to review your sourced costings for you once you get them from wherever in the world and make sure you aren't getting ripped off, assuming you are willing to share them.