Here is a link for a 15T cog. They are very inexpensive.
http://www.amazon.com/Sturmey-Archer...ster+brake+cog
They sell either 1/8" or 3/32" cogs. You can use either but the 1/8" is more durable. Plus you may have a 1/8" chain there already.
All you need is a small flat blade screw driver to swap the cog. There is a snap ring that holds the cog. There are some videos on youtube (the same as swap 3-sped IGH cogs).
Here is one of them:
http://www.youtube.com/watch?v=dxC-9NYrElo