It is not easy to get live prawns. I do almost all my shopping at wetmarkets and certainly neither china town or tekka market sell them.
I did find a place this week though that has them.
It is a small fish shop located on th ecorner of clementi road and pasir panjang road in the west of the island. Beware the prices are a lot more expensive.
For live tiger prawns, the asking price is $25/kilo. For the same size dead prawn at Tekka market you will pay approx $14/kilo.
The time to visit this shop is late morning as the ships with livve seafood arrive at a different time to the normal fishing boats that arrive in the middle of the night.
FYI, this place also has live fish for sale and live flower crabs. the also have live grouper and catfish as well as clams and scallops.
Well worth a visit in my opinion just to get truly fresh live seafood.