You need a 65cm one, as the smaller ones may not support the heft of a pg lady bouncing around in the later weeks. I got mine from Argos for £8 a couple of weeks ago. It's a Reebok one and comes with an adaptor to attach it to a bicycle pump. You'll get fit just be blowing it up! Argos do a cheaper one, but is smaller and has no bike adaptor.
I got mine from Tesco, it was about £6 in their sale. Came with a pump too, it's lovely! I wish I had got a 65 cm one (mine is 55 cm) as although I'm only 5 foot 3, I could have done with being perched a bit higher. They are ace though!