H&M is probably best in store Mrs Jay - they are about £10 for plain short sleeved ones and £20 for long sleeved patterned ones. Found in the maternity section and sized in S, M, L & XL.
However, in my experience, I really prefer to breastfeed in public (all hangs out at home!) in a normal top with a vest top underneath. Pop the boob out of the bra, out of the top of the vest top and pull your normal top up. That way your tummy is covered (very important!) and the normal top covers most of your boob and top of baby's head. I find the nursing tops, although they cover your tummy, the whole boob pops out and its really embarrassing and there's nothing to cover it. I have big boobs though, maybe this isn't an issue if you have a smaller bust and don't have this pendulous breast hanging around!
Hope this helps
Valentine Xxx