If you go to Crufts or know someone that does you can buy it for a good price there - I know a lot of people who buy a years supply there each year - as buying from the vets if you have several dogs can get very expensive (can't remember the name of them - but they also sell online).
Some Vets also do not necessarily offer you the best wormer for your dog - they will offer what is a good deal for them
We use a herbal wormer instead of tablet form, it was recommended by our breeder who has used it for the last 10 years and by several friends who have used it for a good while also
You have to give it two consectutive days each month and for having three dogs its costing us £9 a year to worm all three with it
It can also be given to cats