I got one of those balls at your weight, and they are fine! Often, they come with DVDs that have exercises on them. I couldn't do all the exercises, but it gave me some good ideas.
A lot of people suggest you just SIT on them instead of a regular chair. The idea is that they engage your "core" -- the muscles in your abs for instance -- that help keep you stable. Thus, the name, "Stability Ball". Sitting on them might be a start, at least!
You can buy them anywhere -- Bed Bath and Beyond, Target, Sam's Club, etc...
My 5 C's of healthy living: Commitment to conscious control, with the understanding that choices have consequences