It's pretty easy to find on line. Because it's an ingredient in Thai foods, though, you might check the Broadway Supermarket at 4879 N Broadway. They have a wide variety of Asian ingredients. I know for sure that they have tapioca balls. I don't know about starch, though.
This website has a list of other Thai grocery stores in Chicago. Some of the stores are in the same neighborhood as Broadway Supermarket: http://ammyweb.spaces.live.com/blog/c...
It's really easy to find in asian grocery stores. Just go to Chinatown and stop by Mayflower (or any other grocery store) and get it. It'll be with all the other starches and flours. TH ekind I buy is in a red and white box and it has chinese on it and english on the other side. It's called "tai bai fen" in chinese.