Try Monster Camera (also called Lens.sg) in Peninsular. It is the corner shop across from the foodcourt, beside P&G.
I have tried Hercules, it is made from polycarbonate (plastic), and I personally hated it. But had to use it in a pinch because the one for my camera was totally out of stock at that time and I had a trip coming, so used the hercules for the trip, came back, tore it off and got the GGS installed.