What part of the leg broke? If it's the aluminum portion you can just stop by any old upholstery shop and they can cut a new leg. If it's the plastic piece that holds it to the seat, they are available through McMaster Carr. I have the part number around here somewhere......
1173A21
Plastic Folding-Table Leg Brace for 7/8" Tube (.78" ID), 1-5/8" Square, Black
In stock at $3.09 Each