Here's what I did:
I bought a small and medium t-shirt (the same shirt). I cut the top off the small and pinned the dress together where I had cut the top and sleeves off.
I recommend using elastic thread to sew the top and right under the bust. Here are a couple of links where I learned about sewing with elastic thread, here and here.
Although you might want to use regular elastic for the top. I ended up sewing mine tight enough to where I didn't really even need elastic at the top.
Then I cut the bottom of the medium shirt and attached it to the bottom of the small top. I would recommend just buying two shirts of the same size. I had to resize the med. bottom to fit the small top. Make sense?
Mine did NOT turn out perfect by any means. I honestly had no idea what I was doing and used my seam ripper more than I care to rememer.
Visit the Polka Dot Alley if you want to have one custom made for you. They are constantly adding new pictures of the custom dresses that they have made. They also sell a ton of other cute gift items including a great customizable melamine tray that I really want!
You can also visit Ketch the Spirit. This company was started by a girl that graduated from my alma mater. She only makes dresses for a few university. Check her out to see if she makes one for your school of choice.