Fresh crabs should feel heavy for their size. You can buy a crab live and boil it yourself; alternatively, ask your fishmonger to kill it for you, or buy it ready-cooked. If buying cooked crabmeat ...
If you’re not happy to kill the crab yourself then ask your fishmonger to do this for you. Bring a large pan of salted water to the boil and drop the crab in ensuring it is fully submerged.