Finding under $20 parts retail can be tough because there's so little room for profit. Most bikes you'd put a part like that on already have it. Your LBS might have one in the parts bin. A local co-op would probably have a pile of them somewhere. Otherwise, there's this:

http://www.niagaracycle.com/categori...?sort=priceasc

...one of the few online retailers I know that carries low end parts.