My sis and I have been looking all over for custom 3D cakes. Yes, they do sell these in Singapore and it costs upwards of $100. Always check for delivery as you might accidentally smoosh the nice cake on your way home.
Eatzi Gourmet - Halal bakery, 3D cakes are built on request.
Ecreative - Huge gallery of 3D cakes to choose from with standard pricing.
Cake Avenue - They make bag cakes and other naughty cakes.
The 7th Manna - I only saw 1 3D cake in the gallery, a traditional chinese peach.
Secret Recipe - Limited designs, but, good cake.