The police can get information about who bought diet pills in many ways. First, they can investigate the sales records of pharmacies or online stores to determine whether anyone has bought diet pills. Secondly, they can track the bank or credit card transaction records to see if anyone bought diet pills at a specific time. At the same time, the police can also track the whereabouts of suspects by monitoring cameras or collecting other evidence. These investigation and investigation methods can help the police to determine who bought the diet pills and further carry out relevant investigations.