get the marineland stealth heater (or any other adjustable heater, the hydor theo would work too) from your local petstore or online at http://fish-supplies.fosterandsmithaquatics.com/search?p=Q&lbc=drsfsaquatics&uid=587619683&w=adjus table%20heater&af=pb:02%20cat1:fish%20type:product &isort=score&method=and&ts=results
It'll cost around $20 online and anywhere from $20-40 in the store. I suggest buying from online.
Don't pick the very last one. I had that one and it's crap.
If you can't order stuff online I suggest asking a friend or family member that can. Chances are they'll be happy to do so for you if you hand them your own real cash and watch them order it with their e-cash (credit card, debit card, paypal, what have you).
This way you'll have the heater and your betta wont get stressed and sick from the constant temperature fluctuation and cold temps. =]