These days, everything can be bought online. Buying t-shirts online not only saves you money compared to traditional retail outlets, it also allows you to browse a much wider selection of t-shirts than you could normally find on store shelves.
Threadless has some of the most well designed shirts on the web today. Top notch designers submit their art to Threadless in hopes that it will get printed on T-Shirts. Shirts here are a bit pricey, but you're guaranteed to get something worthwhile and beautiful.
http://www.threadless.com/
For the geek in the family, ThinkGeek is a great place to get nerdy t-shirts. All of the shirts at ThinkGeek are funny and geeky and are sure to bring laughter to others who understand the nerdy humor.
http://www.thinkgeek.com/tshirts-apparel/
Another site similar to Threadless, DesignByHumans is a t-shirt haven for budding designers. The majority of the shirts available at DbH are intricate and elaborate in their color and design schemes. Again, you'll pay a premium for these pretty shirts, but occasionally you can drop in for a sale and only pay $10-15 for these great Tees.
http://www.designbyhumans.com/
The internet is chock full of humorous videos and fads. BustedTees commemorates those fads in t-shirt form so you can be constantly reminded of that funny video or saying you came across years ago. Busted constantly updates their stock, so you're sure to find something you like.
http://www.bustedtees.com/
Here's a name you don't see thrown around too often. Ugmonk is a relatively small t-shirt company that specializes in minimalist designs. If the grand designs available at DbH and Threadless aren't your style, Ugmonk won't disappoint. All of the shirts are based on minimalist philosophies and the color schemes are to die for. Shirts are in the $15-20 range here.
http://shop.ugmonk.com/
select one here...