http://www.ava.gov.sg/AgricultureFisher ... sheryPort/
They don't seem to be too strict regarding the annual fee as I've never paid it. You leave your NRIC/EP at the gate and sign in and off you go, if you have a local friend, I'd suggest getting them to tag along to assist in the "bargaining" process.
Or, you can go to Sheng Siong Supermarkets. They usually have a pretty wide range of live seafoods and crabs. When buying crabs, it's generally best to buy them during the week before the full moon as that will be when the crabs are of fullest prior to moulting to a larger shell. Are you looking for Sea (Flower) Crabs (normally used for Crab Curry or deepfried crab (Indian style) or Mangrove Crabs (those normally used for Chili Crab).
Or are you looking for crabs for steaming Chesapeake Bay style!
Another alternative is buying from a distributor like Jim Foods Supplier
, who I also use now & again as he's very close to my home. But must get there early in the morning as well (7~8 am - call to find out his opening hours).