Water is typically colder than the room it's in. If your room is about 75* then the water will be closer to 70*. That's my usual experience anyway.
I'd get that heater ASAP.
Heaters are the most important part of keeping a betta (next to water and water conditioner).
A typical 25 watt or 50 watt adjustable heater will run you about $20-$30.
Some brands to consider are the Marineland Stealth Pro (or just the regular Stealth if you can find it anywhere) and the Hydor Theo. any truly adjustable heater will do though.
What I mean by "truly adjustable" is that the heater should have a knob/dial you can use to set the temperature and numbers to indicate how high you're setting it. Heaters without knobs/dials usually don't heat the water too well in my experience.
Here are some examples
of the heaters that I mentioned