You can always get a basic hat and add things like flowers, fur,
feathers, jewels, beads, etc etc to jazz it up. That gives it
a unique touch. If you want to go this craft-y way, you can
easily get the things you'll need at Spotlight, which I believe is
still the largest haberdashery in Singapore.
I don't think dressy hats are easy to find in Singapore simply
because, well, I don't remember seeing anyone in Singapore walking
around wearing hats!
BTW, are you thinking of wearing them in some special
function? Here in Melbourne, once a year during the races,
women deck themselves in all manner of ridiculous hats and
fascinators in their hair. You can often get a good laugh
from it. 
