The best price I found when I was looking for bute was pet drugs online but be warned they don't send recorded delivery and when they got the address wrong on my package they were quite unhelpful.
They said they could not send it recorded delivery as it was sent second class (not true you can added recorded for 95p).
They said they would send it out again if it turned up back at their which it did but I got the impression that had it vanished they weren't going to do anything. As it was I ended up waiting three weeks.
My vets have a policy that they do not send out any drugs or even a prescription without a signature needed and I made the mistake of assuming any company would have the same responsible attitude to prescription only medicines.