http://www.statelinetack.com/item/aegis-junior-pony-helmet/E000334/
Here's the link for it at SLT. I can't click on the reviews, so I have no idea why it averaged out to a 2 I'm pretty happy with mine. Stays on DD's head nicely.
Here's their shipping info. Didn't see Mexico, but they talk...
I got an Aegis Youth for my daughter. It is adjustable. IIRC, it was about $50 from State Line Tack catalog. Dover, Miller's and others probably carry it too.
Here is DD wearing it while going into dressage leadline on my friend's horse