There is no official merch being sold internationally (to my knowledge) but if you want to help him out there is a donation box in the official website for his political party: www.nuevasideas.org

And given the circumstances that he's facing and all the crap that the media is at throwing at him, I'm sure any help can be benefitial.

The situation is not looking good down here and he's running against 2 big corrupt giants. I'm not saying he's gonna solve all the problems in our country but he's the best we have right now. And if you want to do something from the outside you can inform yourself about the real issues the country is facing and what the candidates are willing to do about it and spread the word. Sorry for turning this response into something political.