There aren't any specialty stores that I know of, however there are hobby-type stores located inside the malls. I went to ShoeMart Megamall and saw one there. Actually, it may even be a Tamiya store that sells aviation stuff if I remember correctly. Be warned, however, they do cost a lot over there. You may be better off buying elsewhere.
How long will you be there? Email me at firstname.lastname@example.org if you want more info on Manila. I just came from there a couple of months ago.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!