Amazon do them, they're called Rain Ponchos. I can't actually get on Amazon at the moment though to post the link
Thanks folks.
I need a couple before Sunday so Amazon won't do. I need them for a concert next Tuesday and the forecast isn't brilliant
If its an outdoor concert you will get them outside of the venue. I saw Coldplay at Sunderland a couple of weeks ago and there were loads of people selling them outside for £2 each inside they were £3