Betta in a bowl needs a filter, what are my options?

I'm reading your question while my betta is swimming in his 1.5 gallon tank right beside me. I have one of those already set up all in one tanks that comes with a small air pump powered filter. You can buy these filters separately or just buy the tank (I got mine for 20-25 bucks at walmart), although its not as big as a 2.5 gallon:(