The ones I've seen - and I don't know if they are the only ones or not - are by the ladies' toilets and the sweet stall in the middle of the concourse, just to the right (facing the pitch) of the entrance to Block G.
Oh FFS - what am I supposed to do with the bag of approximately 10,000 bottle tops I've collected, enabling me to smuggle a couple into the Amex for each fixture*?
(* - when I remember, which I don't always. And there's no angrier site in football than me rocking up at the WSU entrance and...