Heart worm pills

Lady Violet is about 9 months old now. I'm curious about heartworm prevention pills. When should a dog go on them? Rascal was on them all the time in Michigan because there were so many mosquitoes but I never gave him any pills in CA. I brought Violet to the vet a little over a month ago and they said nothing about putting her on it. No vet has ever said anything about it here actually in the five years I've had a dog here. What does everyone think- do dogs need to be on heartworm prevention pills in CA even though there are very few mosquitoes?

What's the best website to purchase dog drugs?