Just make sure its big enough and as one too small will be worse for your hips. Worth looking at the anti birst gym balls as well as specific birth balls as that's the only difference from a standard gym ball to a birth ball.
Argos, prices £4-£20 ish. They are sold a gym balls, but am told by pregnancy yoga instructor that they are they same. Most of the Argos ones are anti-burst, or or burst proof up to quite heavy weights. They also have various diameters - 55, 65, and 75 cm.