I am a large animal vet dealing with mainly horses and farm animals so not familiar with that product, however it will be more from your vet because they will be buying it in small amounts from a supplier rather than in bulk and they will have a mark up on it which varies between practices.
You can always ask for a written prescription which can be used to buy medication from any licensed pharmacy. I would be very careful with some of the online drugs sites as there are some suspect ones e.g. selling medications cheaply with a short date on for instance. Most practices will charge a small prescription fee for sorting out the paper work.
Whilst your vet is likely to be slightly more expensive you do at least have the security of knowing where the medication has come from and a ready source of on tap advice.
Hope this helps!