best ones are prob vet prescribed ones so if your bun registered at a practice for jabs etc should be able to go n pick some up.
advantage 40 dog is licensed for buns as is xeno 450,the new ivermectin spot on so even if its not fleas but another parasite that'll kill em for sure.
tend not to bother with pet shop stuff like bob martin as all their dog/cat stuff is pants! you just end up shelling out twice to then get something effective...