Daily Steals offers the PharMeDoc Balance Ball Chair in Black for $69.99. Coupon code "DSBBCHAIR" cuts it to $50. With free shipping, that's the lowest price we could find by $5. This 52cm ball supports up to 300 lbs., is made of anti-burst PVC, and features a removable back rest. A pump is included.
WD-40 is handy in a lot more situations than you likely realize.