Almost every DB vendor offers some sample DB for download, e.g.
Then you can download the QV tutorial, which contains a simple model.
This QV book comes with a sample flight database:
There are myriads of data sources in the web you can access (by data file download or HTML access in the QV script).
Without knowing the domain you are searching for, the amount of possible DBs is pretty large ;-)
If you want to download SQL Server databases then use below links, likewise you have many default databases for all the databases that comes with installation. You can search in google for the database you need
Here is a sample database to get started
this is one that is used for Crystal Reports
xtreme.mdb 10.8 MB