i think bumps to babes may have them. although mothercare had something more like an angle (like a triangle with one side gone) which i used up in the matilda and found it great.
if you fancy one like the one meridien sells (they also rent medela pumps so you can see on their website) i have one which i can pass to you for free. just email if you want it. it doesn't wrap around the waist, but you can use it until you find a boppy.