For those who are still interested in buying plastic sleeves in Quiapo...
I was able to pass by the area last weekend, the store is along Elizondo, look for the green building (it's on the left side). If you are coming from the barter center, you need to cross Arlegui.
The only size that they have is for regular amaray cases (6x9 i guess), and it cost P50/100pcs.