my mare was prescribed ventipulmin and after 1 months worth we decided to try something else and swapped to breathe easy from supplementsolutions.co.uk and have never looked back since. it is a 1/3 the price and does a far better job
Try googling Beeston Animal Health, I got some last year from them, still not cheap but £17 per tub cheaper than my Vet and received it a couple of days after I sent them the script.