yes you can use flexwatt throughout the entire life of the snake. Under the tank or on the side is up to you. I have two racks that use back heat instead of belly and work great for smaller racks or tanks. If the room the snake is being kept in isn't temp controlled belly heat may be the best option.
I'm at work/on phone so can't look up links but google "LLL reptile+heat tape" should come up with the link. I buy most of mine through them.