Peak Flow
Motions, you can indeed buy one over the counter as Mcken has posted. I've just purchased one over the counter in Boots. I was offered two versions - one was smaller than the other. I opted for the larger model as it looks very similar to the one my AME makes me use. It cost me £12. If you wanted to, it would cost you £6.75 if you asked your GP to put one on perscription for you.
The one I went for was called the Mini Wright and is made by Clement Clarke International. Part No EN 13826.