Single speed cog in rear
 

I am looking to buy single sped cog..either a 11 12 or 13t. Does anyone know of a place to get them.

If you mean a singlespeed freewheel, they don't come that small unfortunately. You're going to have to get a bigger chaining instead to work around that problem.

Or alternatively, get a freewheel hub from a road bike, assuming you have a compatible frame, and use a smaller casette cog with spacers.

Fixed cogs aren't readily available below 12T either (Phil makes an 11T, but it requires a special lockring for chain clearance).

Thanks guys.