Generally, the purchase of drugs in Singapore is licensed under the Poisons Act. So unless you are buying them off the dark net, then no, there is no way you can just purchase it. And of course I'm just kidding, please don't go to the dark net.
Otherwise, your underlying medical condition needs to be treated.
Medications like benzodiazepines cause more problems than they treat in the long term. Please see a mental health professional.