Make sure you never get something called 'haylage' which is something that's fed to horses. I doubt that would be suitable for guinea pigs as it's moist, and quite sweet and a little sticky. I only point that out, because at our stables we just call it 'hay', so they may give you it by mistake.
@Princess_Piggie Hey! Sorry it took so long for me to see this. Yes, I am from the Philippines! So far I haven't found any good store in Makati that sells hay. Actually, it's SUPER hard to find hay in the Philippines so I get my hay both online and in stores. If you're going to buy online, try (broken link removed) and simply type in timothy hay for guinea pigs. A lot of sellers actually sell them for good prices and they sell those in good quality too (they also sell timothy based pellets, since a lot of feeds being sold in this country are alfalfa based or a mix of grains and seeds, just a tip ). As for the stores, I recommend the Sea and Land Pet Shop in Shopwise Cubao, or the several stores in Cartimar. It's easy to see the hay in Cartimar stores because they're all stored in one area. It's also easier to buy from here since hay is available all year round, unlike some pet stores in the Metro Manila area. Gotta warn you though, lots of stores in Cartimar sell live guinea pigs so choose the stores that only sell pet supplies and no live pets!