-You can get iron-on letters and designs from fabric/craft stores.
-There are fabric markers; basically sharpies specifically made to stay on a shirt. These should be available at an art supply store or craft store.
-You could rock it 90s style and get puff paints. If you're doing puff paints make sure you follow the instructions for using them on fabric. If you don't they will peel off the t-shirts you're decorating really easily.
-If you're lucky enough to have a friend or relative that does screen printing, it's relatively easy to make a design and inexpensive as long as you can use someone else's materials and experience.
-You can get random things from a craft store and get some fabric glue to adhere things like sequins or other small objects to the t-shirts. Just read the labels and make sure they were made for fabric and will hold fast if washed.
-You can look at thrift stores for cheap, unique t-shirts and add to them with some of the above techniques.
I love cropped sweaters! I just recently got one in a floral design!
Forever 21 has some like this one: http://www.forever21.com/Product/Category.aspx?br=f21&category=promo-cropped-sweaters (its pink).
I found a light greenish cropped sweater at Zara: http://www.zara.com/us/en/sale/woman/knitwear/sweaters/cropped-sweater-c541559p1666666.html
The first comment said Hollister and I found two good ones there though they are grey:
Thats all I could find for now.
I hope this helps (: