You probably don't see any pet shop selling mantis in Singapore because you can find them in the wild pretty often. Having a praying mantis as a pet is still not a popular thing in South East Asia, people there are more interested in pets like dogs and cats, even beetles or lizards are more popular than insects in these countries. I live in Malaysia for the first 20 yrs of my life, then, people think keeping bugs is nut. But it is slowly changing now, mainly due to the demands overseas. Anyway, sorry i have no idea where you can find one in Singapore. But i am sure you can find them outdoor if you look carefully in any botanical gardens or parks.