The best quality t-shirts are done by screen printing. So your best bet is to find a decent screen printer and ask them for a quote. But keep in mind that 4 color (CMYK) shirts generally don't look good and most screen printers will do what they call a "process color simulation" where they separate the color image into 6-10 custom colors. They may also run two whites, one under the entire design and one to strengthen the highlight colors.
I have been working for 5 months at a screen printer, and I am learning more about the process every day. The biggest thing to keep in mind is that you will have screen setup charges, and the more shirts you have printed, the better the price per shirt will get.
What are you looking to have printed? Can you post an image of it?