Hi luis,
In South America, there is a distributor of Strida in Chile. You can ask them whether they are willing to ship parts to your country. 
http://www.beldabikes.com/STRIDA_center.asp
Interetsingly, the distributor of Strida in the USA has a Spanish website. Maybe they can ship parts to South America, too.
http://www.strida.us/sp/default.asp
If the distributors in Chile and the USA cannot help you, then you can ask the Mr.
in Hong Kong. 
[url]ChainWheel replacement for Strida 5 ? - #6 by Blackstridaaustria]
Regards,
Amuro