I haven't seen fire Pok?mon at gas stations yet. Maybe they are. Water types are found near water, normal types in the city, bug and grass in parks and grassy area, ghost types at night, etc. I don't know exactly where every type is most likely to be found. They can be found everywhere, especially with lures at pokestops and if there's a lot of pokestops nearby with a lot of people and a lot of lures. That's when you'll find rare Pok?mon.