As already stated, the sample databases area on this and other forums is a good starting point.
However it might make more sense to look for databases related to specific subjects that interest you.
needs some context.
Example(s) of
Database general,
Database Concepts
Project requirements,
Analysis and Design
Business Processes
and the list goes on...
Do NOT overlook the value of Google or Bing or Youtube to research any topic.
Examples are all over the place, starting with the templates provided by MS. They may not all be 'great' and some even do things I won't but can be a source of ideas. The Northwind template is one. It has been used in tutorials too numerous to count.