As other have said builders merchant. The problem is they don't always have everything on display and if your not sure of the exact name, it can sometimes be a pain.

If you're looking for a store where you can see everything on display, I would recommend Wilcon Builders Depot. Not always the cheapest but you can at least find what you want without knowing the proper name. They also have a very good range of tiles and indoor stuffs. I know there is one on C5 just before the turn off for Marakina, as I pass it at least twice a week. Wouldn't cost much to go there by taxi.

http://www.wilcon.com.ph/