I finally found the right size tire at my Saturn dealer, of all places. I did a google search for the size (T115/70R14) and saw that some Saturns use the same size spare. My other car is a Saturn wagon, so I called my dealer and they had the tire in stock. The Insight sure got a lot of attention at Saturn of Decatur! The service person wanted to write down the odometer reading for her form, so I got to show her the dashboard lighting up and the mpg display.
The tire's been on for about a week now, and seems to be holding pressure just fine.