For all those clients I have recommended use posture sitting ball, below are a few links to purchase them.
http://www.pilates-mad.com/product/7-exer-soft-ball-blue
http://www.physiosupplies.com/acatalog/Exersoft_Balls.html
http://www.amazon.co.uk/Gymnic-Soft-Gym-Exercise-Ball/dp/B0043H2YWI/ref=sr_1_7?ie=UTF8&qid=1308350699&sr=8-7
Essentially you are looking for soft pilates type exercise ball of around 7inches. Please let me know if you find them for an amazing price somewhere so I can share it with other clients.
