Major dept stores have. If not try, bricksworld. They are a lego specialist chain of shops. There's 1 in Plaza Singapura Level 3 and 1 in Ngee Ann City on the same floor as Best Denki, Level 5 I think.
Price depends on how big a set you choose. I think the basic ones start from about $20+ for a small container. I bought the Duplo Ville sets for my gal when she was about that age. More interesting cos there's the characters and building sets as well as vehicles.
One of the reasons why Lego is very expensive is bcos of their technology. You can buy 10 - 20 boxes of lego bricks and from different batches, but all of their bricks still clip onto one another very properly and without protuding edges, unlike some others.
For a cheaper alternative, go for Megabloks. Also avail in mega dept stores and got syndicated characters like Dora, Disney Princess, etc too. But I personally feel that Megabloks bricks dont't hold together as well as Lego.