Lots of Yamaha shops around. There are Yamaha stores at PS and
Tampines Mall, among others.
From what I've found out from the Yamaha salespeople, PSR-E203
($250) is a good keyboard for beginners. I think what your friend
bought was PSR-E303 ($350). If you're going to learn to play the
piano later on you might want to go for the E303. It has
touch-sensitive keys. The loudness of the tune will depend on how
hard you press the key. That's important if you want to learn the
piano later on.
But if you're just sticking to the keyboard, then just go for the
cheaper E203.
