Small tanks or fish bowls can be a real challenge to heat properly. Fortunately there small heaters are available in a variety of types. Check out these heaters specifically designed for mini aquairums or bowls.
Avaialble in 7.5 and 15 watt sizes, this flat, fully submersible heater is a great choice for a small tank. It fits flat against the side of the tank, or better yet, under the gravel bed. The price is also fairly small. Ten dollars will get you the smaller version, which is perfect for a Betta tank or bowl.
Marineland makes solid products, and this one is no exception. It is a mere 5 inches long, shatterproof, preset to 78 degrees, and has an automatic saftey shut-off if not submerged in water. Costs a bit more than the flat heaters, but it's a quality product.