Hi the cheapest I have found by doing a quick search was on Window cleaning warehouse for £3.75 each.
I am sure though you could try emailing them or other internet distributors like soap national, cleantech, cleaning technicians, wintecs or scrimcity let them know you want to buy 50 and see what if any discount they would give you for buying 50.
Other idea is if you are a member of the Federation see what deal they can do for you after all there are advantages to being a member.
Good luck