Jan-
What's the problem with controlling training materials that have been purchased? You control external standards and customer supplied documents....dont you? Control purchased outside training materials in the same way.
When anyone asks me if a particular document needs to be controlled, I always ask them:
1) Does the document need to be accurate?
2) Will/could the document be revised?
3) Is it important that obsolete versions are removed?
4) Does the docuement need to be distributed to certain people in a timely (business days) manner?
5) If the document changes, and the changes do not get relayed to everyone who uses the document, could it result in a non-conformance?
If the answer is "yes" to any of these questions, the docuemnt msut be controlled. The word "controlled" is narrow interpreted as a document that goes on a master list. In addition it can be misleading amd intimdating to the people who create/use the document. These people sometimes feel that QA is trying to "control" what they do. Make sure that you explan to them that you just want to make sure the document is accurate, and gets to the right, people when they need it. That is the essence of document control.