We use a holistic wormer for Corbie called Four Seasons - only costs just under £8 for a bottle which I order online and contains enough wormer for 4 dogs for one year. The vet was ok with it when i told her what I use.
We've had no problems with it and after my parents' dogs both had side effects from the chemical wormer their vet gave them (mum can't remember the name of it), I would much rather use the holistic version on Corbie. He has this every month.
just wanted to say if you do find a pet store rather than go to your vets, beware of some 'worming' products (Bob mortimer to name one) as they do not get rid of all worms. (so i have been told anyway) :wink: