you are not allow to keep snake as pet in local, so you can't find them in any pet shops.
there are few ways you can try;
you can visit some small Chinese temples, they may have some snakes kept there, ask them can you borrow their snakes for a day or two.
or you can hang out areas like Mandai reservoir, you might able to catch a snake or two.
alternative, you can use eel instead, which you can buy from Chinatown market easily.
last resort, you can buy the rubble snakes from Toy "R" us, if the models are afraid of snakes.
hope this help.