I get Frontline at our vet, but most Pet shops also have them. I also get the anti-heart worm tabs at our vet. (Not sure if its called Heartguard, thats why I didnt specify the name )
We get them at Pet's Veternaria next to the Comercial Mexicana on Tulum. They do grooming too (thank goodness, I cannot bathe my hyper long haired mutt!~)
Heartguard is more commonly known as Cardomex here now. I was looking for heartguard for a friend from the States and couldn't find it. But Cardomex is the same stuff. That and Frontline can be found at any petshop.