Hey Jen -
sorry I missed this until now... I weigh a little more than you (I think) and have had NO issue with ANY exercise ball holding my weight. I was a little nervous at first, but now I use one all the time at the gym to do crunches with!
I think you can get them at Target, Walmart... just about anywhere now. I forget.. I think you are in Canada- So now I have 2 friends named Jen in Canada HA! anyway, I would imagine you can find them up there too!