There is a store in SM Makati and SM Megamall called Surplus Shop. The store carries american labels like urban outfitters, The Gap, abercrombie, ralph lauren etc. But these are excess from the export bins in the philippine factories. The clothes are in good condition, properly labelled and would cost you an average of P300-P600 pesos for jackets and shirts.
You may also try Rustans department store.