Where can I find ready-to-use database models ?
I don't need a database with data in it, but only schemas (UML diagrams). Perhaps something like the data models at this link, but much more complex and real world.
Michael Blaha's Patterns of Data Modeling. This one has some interesting temporal, graph, and tree models.
Martin Fowler's Analysis Patterns. This one skims some of the other patterns, but gives accounting a solid treatment.
They are all well-rated, and I have read all but one, and they are all very good. Several of them are available on safaribooksonline.
If it's more complex than the free data models you've mentioned, then I'd suggest it is a custom model for your requirements.
Anything complex would most likely be commercial, proprietary, or otherwise someone's IP.
For example, a data model for an MMORPG or a derivatives trading system.
What are you trying to do anyway?