Buying different heaters for each tank would be very expensive. You could always buy a space heater and heat one small room in your house up to around 78 degrees. This might be cheaper than 5 proper heaters.
I suggest if you cannot afford larger tanks, to get clear plastic rubbermaid/sterilite storage bins. You can get 5 gallon bins for only about $4, these are perfectly safe to heat, and very cheap. They are ideal for bettas because they are light and durable for water changes, and they are long and shallow.
The only heaters I can recommend are adjustable ones. They are only a little more expensive but they are infinitely more valuable--if the air is particularly cold, non adjustables will not get hot enough anyway. If you are ordering multiple heaters I strongly encourage you to shop online--it will be MUCH cheaper, http://www.fosterandsmithaquatics.com
offers flat rate shipping, and the adjustable 25 watt heaters are very well-priced, if you found these at a regular pet store you would pay $10 more EACH, so even with shipping, if you buy these online, you save approximately $40.
In the future, you should get two ten gallon tanks, divide one three ways and one two ways. This would be the easiest and cheapest way to maintain all your fish in a heated, cycled environment. You should save your money with this as your goal. I encourage you not to get any more fish until you can house the ones you have properly.