Most hardware stores should have it.
Which part of Singapore do you stay? If you staying in the east side, you can try Blk 824, Tampines New Town (opposite Temasek Poly) that's where I bought mine from.
You can also go to Eunos Ave 5 (industrial area), there're couple of shops at blk 1035 or thereabouts that carries PVC pipes.
Cheers!
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