I have found just the web D ring reins for pretty cheap (altho horse sized, I recall Jeffers catalog carries them) and you can buy white cord and snaps at the hardware store and tie the rest of the martingale to fit the pony. I suppose you could cut the reins down too.
I actually prefer the cord b/c it runs thru the bit better. Much better than the thick leather on the martingales you buy.
Some day I'm going to tell my grandkids that I am older than the internet and blow their minds.