You can have fun with the data in your personal dev store, however any data that could potentially be seen by the client should either be real examples of their data, or close representations of it.
Use real data examples wherever possible.
Request sample data from your Account Manager, Project Manager, or Client.
Ensure you have multiple forms/sets of data to work with.
Ensure your test data covers all your needs for testing the project requirements.
Ensure your data is up to date, and refresh it if necessary.