I have consulted with my wife who has been teaching Filipino culture for 16 years, and Morningstar, I am sorry to say, the rainstick is not indigenous to the Philippines. The ancestors of Filipinos, the Negritoes, the Indonesians, the Malay, and the Chinese do not have such an instrument in their history. More than likely it was introduced by the Spanish. The origins of the rainstick as a musical instrument is the stick or tubular rattle. Researchers at the turn of the 19th century describe tubular rattles very similar to the rainstick which were made and used by the Cuna of Panama; the Colorado of Ecuador; the Macushí, Uachmiri, and Yauapery of northern Amazonia; and the Huichol of northern Mexico. There are also wooden stick rattles filled with seeds or pebbles found archaeologically along the coast of Peru, but it is not clear whether these date from the Inca period or earlier (6000 B.C. - A.D. 1500). Some scholars and Mexican musicians suggest that the rainstick was introduced into popular music in that country from South America. Some say the popularity of Andean music, which uses the rainstick, fostered its adoption. Other people suggest that the ecology movement to save the Amazon rainforests, which began in the 1960s, brought the rainstick forward as a symbol for the rainforest. Whatever the case, the rainstick has become a popular musical instrument and has been adopted by many musicians around the world. Needless to say, just as the Fiestas which are not indigenous to the Philippines, but were introduced by the Spanish, the rainstick, too, is meant for the tourists. They can be purchased at Kultura in the SM Malls, at Sulit.com.ph and on eBay, among many other places. The project would be fun for the kids, so I wish you well with your lesson plan.
Sources: http://www.philippine-history.org/early-filipinos.htm,
http://answers.yahoo.com/question/index?qid=20080424202536AAkXwhr,
http://en.wikipedia.org/wiki/RainstickPhilippine History Page on our Parent Site: http://www.bjruth.com/A_Little_History.html