Zazzle are having a sale on their Star Wars t-shirts at the moment with a nice big 30% off if you use the coupon SWTSHIRTSALE. Zazzle is an officially licensed vendor of Star Wars gear and there is a pretty huge selection of designs and t-shirt styles to choose from. On top of that the designs at Zazzle are usually customizable so you can make your Star Wars tee unique. The sale ends August 22nd.
Of course is you are looking for something a bit different you can check out this previous list of Star Wars T-Shirts.