I heard Microsoft allows use of commercially available Office UI controls, with the exception of competing products, such as a word processor or spreadsheet app, etc.
How true is that?
Also, if it is not true, do you know of any free Ribbon controls?
You should look at Jensen Harris' blog entry about licensing the Office user interface which explains it in great detail. However I believe the relevant point here is:
There's only one limitation: if you are building a program which directly competes with Word, Excel, PowerPoint, Outlook, or Access (the Microsoft applications with the new UI), you can't obtain the royalty-free license.