I bought a couple of vests in New Look the other day and have found that they're the best thing ever for breast feeding (had no idea when I bought them)
They're this new style of a tight-ish under vest with a big long baggy one over the top. I simply lifted the under vest to expose the boob and slipped the outer vest over to cover myself. You'd have to try it but it works and is great. I am a shameless public breast feeder (I kind of forget they're not supposed to be seen by everyone) but this kept me well covered and decent at dd's birthday party and dp's dad didn't have to leave the room bright red when I fed dd.
I think they were either £10 or £15 each and come in loads of colours and wash well - mine line dried and didn't need ironing (well, I didn't iron them )