Toddy, do you want to write the letters in Spanish or English?
If you want to do it in Spanish, follow Nuria's advice. If you want to write them in English, or in both, try contacting any of the schools in the list at this web address:
http://www.embusa.es/edu/listen.html (A word of warning from one teacher to another - the English speaking schools on the list get many requests for pen pals. Good luck!)