The smallest freewheel you'll find to fit an standard hub thread (which is on 99% of all single speed hubs) is 16 tooth. There are several brands, such as Dicta, Shimano etc. expect to pay at least $12.

AC Racing does sell a 15 tooth standard freewheel. They do this by moving the pawls outboard. they sell for around $25, but are somewhat hard to find.

The best thing to do is find a bigger chainring if you want a higher gear