There are many interested groups who study the problem of drug abuse, and their data are generally available on line. Here is a partial list of such sources:
1. Self-halp groups (AA, drug treatment organizations)
2. Law enforcement, DEA, FBI, with annual reporting of statistics
3. Medical publications from emergency services, medical examiner and coroner offices
4. Underground organizations
5. Surveys by pro and anti drug organizations
Each source has its own advantages and disadvantages based on their inherent bias, focus or interest, funding, agenda, etc.
Drug abuse is rampant, and the situation is fluid. Popularity of specific street drugs varies from day to day. It is therefore very difficult to get accurate, timely and current data. The Internet should help. In fact, I would say that the best answer to your question is that the best method of collecting information on drug use is intelligent use of the Internet.