To get started, you may want to consult:
For books, try these subject headings:
You can also use the construction Populism -- (Continent / Country) to look at books dealing with populist movements in particular areas. For example:
In library databases, try combining populism with issues, events, or themes that you think contribute to your response.
You may also want to search for populis* - this will search for populism, populist, populists, etc.
Try thinks like: