Where can I buy threaded fixed-gear cogs (3/32") for a reasonable price?

I recently got a used Cutter Bones singlespeed/fixed gear bike on the cheap for doing errands around town. But the singlespeed freewheel needs replacement, and while I wait for that order to arrive I've switched it over to a fixed gear setup.

Turns out it's kinda fun as a fixed-gear bike, but I want a smaller cog for that (going from 18 teeth to maybe 16). So where do you all get your threaded cogs? I saw that Surly sells some, but for like $30 a piece, which seems excessive. Ideally I'd like to get a set of cogs in the range of 14-17 teeth to experiment and see what works for me, if they can be had cheaper.