Here's a heater that will work:
Aquarium Heaters & Temperature Control: Marineland Shatterproof Heater
And here's a test kit:
Aquarium Water Testing: Aquarium Pharmaceuticals Freshwater Master Test Kit
Both of those things are very important. Without the heater, the betta will be lethargic and just kind of lay about. Also, his immune system will be greatly compromised, which will make him much more susceptible to diseases. You'll also want a thermometer like this one:
That way, you can always know exactly what temperature your betta's water is.
The test kit is also extremely important as it allows you to monitor your water parameters. I urge you to read up on the aquarium cycle (the link Little-Fizz posted has some great info). The temperature and pH, along with your levels of ammonia, nitrite and nitrate, are your most important water parameters so knowing their levels with a good test kit will allow you to do water changes when your fish needs it.
You don't have to buy those items from that particular website, but both the heater and the test kit are going to cost substantially more (I'm talking twice as much) in a pet store than they will at an online retailer.