Gourdo's, Cooks Exchange and Baker's Depot are good and usually have extensive selections of baking and decorating supplies. I find them a bit pricey, though.
I buy my piping tips from Saizen
You can get around 3 metal tips and 6 plastic tips for only 88 pesos there. Of course, they're not as good as the ones in specialty stores, but if you're a casual baker, they should be fine.