Terry brand carries some excellently sized bike jerseys (available at Team Estrogen).
Look for jerseys that don't have a fitted bottom or an elastic bottom. The plus sizes offered by Terry are excellent in this regard (plenty of room in the midriff area). You might also want to check out some of the Mt. Borah tops. They make some in larger sizes that have a bit of ease through the hips. I have found that their sizing is smaller than what is indicated though (eg. a size 18 is more like a 16).
Hope this helps--
Jane
Hey, if *I* can ride my bike every day in Chicago (rain, shine, winter, summer), dragging 100 lbs of kids in a bike trailer, then ANYONE can!!!!