Sorry, been busy. Anyway, MarkIII, if you're from Imus, its close enough. There's a store right on Recto St. that sells rubber foam. It's what I've been using, and costs only Php100 for a 1X2 m sheet.
If you're as well traveled in Manila as I was at your age, you can get to Baclaran, take the LRT and get off a D. Jose station. There's and overhead bridgeway that connects it to the LRT 2 line. Get out on the Recto side and walk towards Divisoria. You're pretty young, so your folk might not let you go on your own, also, not a good idea anyway.
The store I get my foam from is a small shoe supply store, pretty hard to find, but well worth the effort for the cheapness of the stuff.
Next time I go, probably sometime next week, I'll try to get a GPS fix on the place and post it here for you.
Here's a map of the general area. The LRT 1 and LRT 2 stations are linked by an overhead bridgeway, and you can walk from the LRT 2 station to the store marked with the X. Its the general area, you'll need to keep your eyes peeled to find it.