If you have a logo already, there's cafepress.com: upload and hello, shirt.

Looking for something a bit more...professional but thanks =)

What is your goal? What style? I think if you give a few more specifics you might get more suggestions.

Goal: We're looking for something that will catch someone's eye, something unique although highly readable.

Style: An abstract flavor is nice, borderline graffiti maybe. Our colors are green, white, dark grey/black.

We have a logo, not sure if it's the best one as of yet.

hey i could give it a shot...could u furnish more details..

try zazzle

While the eye-catching design is a strategy, what is the end goal? Are you trying to get people to buy your product or service? Do you want them to request information? Are you trying to build name recognition?

Who is your target market? Young urban teens? Educated professionals? Farmers? Techies? Large corporations? Individuals? Small businesses?

How will the t-shirts be distributed? A small run to staff? Give-aways to customers (Free t-shirt with every $100 order), giveaways at tradeshows/events? Sold for profit? How do the t-shirts integrate with your other marketing strategies? Will you want to use this design on other items? Coasters? Keychains? Postcards? Brochures?

How will they be printed? Zazzle and Cafe Press give you more design latitude (in some aspects, less in others), but may be more expensive per shirt than screen printing. If you need many shirts and are going to have them screenprinted, what color shirts will you use and how many ink colors? If you print on a dark color you may need two print runs to do an undercoat of white first, then a hit of color second. This adds to cost. Using this method you will also want to use 1-4 spot colors rather than 4-color process (doesn't work on screen-printed shirts because the dot pattern would be too large).

What are the PMS colors for your green and dark grey?

I don't want to sound overwhelming, but these are the types of issues I consider when developing a marketing plan for any media.