One can begin with:

• Cortellis – Allows to search for drug sales and drug sale forecasts by date range, drug name, etc.
• Statista – Provides quick facts on the topic. You may also notice the source cited  in the data record as a path for tracing further info. 
• Bloomberg -- Drug sales and revenue forecast data. 
• LSEG Workspace: Type HCARE to get to a subset of content from Cortellis.

Additionally:

• SimplyAnalytics - After login, Specify location > select Data tab > select Consumer Behavior > search for "drugs" and select the data of interest. Data can be displayed on a map or in a table.
• OECD Library - often have high level pharmaceutical statistics.
• ScienceDirect - Through literature search, one may find useful datasets from existing studies. This database can be a good place to begin. 
• Data.CMS,gov -- CMS provides some prescription sales/payment data. May begin a search on the website using keywords: drug sales data.