China Square, as suggested, is probably the best place in Singapore to go if you're looking for toys and figurines. It is, however, quite a fair bit away from the usual tourist areas, tucked away as it is in a little-visited corner of the financial district a few streets away. Meaning to say, the place can look a bit... dilapidated. Shady, even, almost like the kind of place you'd go to score a joint (I'm exaggerating. Please, please, please don't do drugs here. It's not worth the risk).
So, if you're expecting something like Mandarake in Shibuya, you'll be disappointed.
You could also try the seventh and top floor of
Plaza Singapura, which sits just on top of Dhoby Ghaut MRT station, the same station you'd arrive in on the way to Sunshine Plaza a few buildings away. There isn't as great a variety of toy shops in Plaza Singapura as in China Square, but you will find one for figurines (Toy Closet/Outpost), one for Hollywood-related toys (Movie Replicas Collection), one dedicated to cosplay (Otaku House) and even one shop dedicated replica mediaeval weapons (Caesars). The aforementioned
Comics Connection manga/toys franchise also has a store on the sixth floor.
Have fun. Hope you enjoy yourself in Singapore.
