I got mine in Argos and it was mega cheap!
https://www.argos.co.uk/static/Product/partNumber/9283574/Trail/searchtext>GYM+BALL.htm
I'm on it right now haha. I find it much more comfortable to sit on than the couch these days, and they're supposed to help the baby get into a good position for labour!
I would just add if you are fairly tall try out a larger one (75cm) they are more expensive but i found the standard (65cm) ball was too small for me - i'm 5'7"
xx