stability balls are available everywhere nowadays, however very few places have the right size for most women… if you are small like me, you need a 55 cm one (and most shops only sell 65 or 75 which would be for someone 6 ft tall).
it is very important that when sitting in the ball your hips are aligned with your knees, and no higher.
As well depending on the quality of the material, the ball will be firm (which is what you are looking for) or soft and sinky!… stay away from Reebok ones…they are far too soft…
i always buy mine from sports providers..there is a little shop at the bottom of stockwell street called Marlis’ and he supplied me with all the ones i had in studio…very cheap …about 15 euro for a proper one…but remember to buy a 55cm.