There is this camping accessories shop (forgot its name) on the 2nd floor of Penisula Plaza, diagionally across K13. You can buy those already made or special order.
It is cheaper to buy from the US if you have friends or relatives there. I bought mine when I was still in the university in New York. My Timbuk2 is still taking the beat from my daily use. I brought it to rafting and rock climbing before. Tough as nail.