Love these types of entrepreneurial questions. First off - always remember that you make most of your margin when you buy, not sell. That said, let's now move on to finding some domestic suppliers.
Here is a lit of suppliers from TopTenWholesale.com that feature quite a few verified wholesalers in the usa: http://www.toptenwholesale.com/wholesale-t%20shirts
If this is something you are dead set on, I'd always recommend attending a trade show. The 3 best shows you might want to try would be:
- The ASD Show, MAGIC , The Off Price Show -- all located in Las Vegas.
If you require additional help, just submit a detail product request on TopTenWholesale and you'll have multiple suppliers getting right back to. There is no charge to do so - or, you can always call them at 1-800-928-top10
That is a highly competitive, saturated field that you might find difficult to make profitable, I recall one guy with original designs who spend $1000 to setup a site and was still waiting (I don't recall how long) to make his first sale.
Do you know of the print on demand services tat can affix an image to a variety of objects, that allow you to setup a 'store' with your designs, setting your own markup prices in some cases.
The big names are cafepress.com, Zazzle.com, spreadshirt.com, Customink.com
The total pages on these sites, mostly representing user store pages: cafepress.com 598 million pages!, Zazzle.com 37.5 million pages, spreadshirt.com 9.9 million pages, Customink.com 0.4 Million pages.
To have your designs printed for you to directly sell there are countless online services that can print on shirts they supply, their wholesale costs may allow them to match the best prices you can get, the cheapest I have seen.
VistaPrints.com $6 offered in a Google Adwords search ad when you search: t-shirt drop ship
Drop shippers deliver directly to your customer in a generic package that doesn't reveal who made it.