NOTE: This information is for Knoppix 4.0.2 CD as I don't have newer. Still I'd quess that it's pretty much the same for 5.0.1 also.

You can pay Klaus to send the sources to you by mail, as mentioned in KNOPPIX/index_en.html file:

You may find the sources for the installed Debian packages on the various Debian mirrors.
Additionally, you can order the sources directly from Knopper.Net for the cost of material,
copying, packaging and postage.
This offer is valid for 3 years, counting from the build date of this CD-Rom/DVD.
Alternatively, you could get sources from http://www.kernel.org/ because according
to /usr/src/knoppix-kernel.README Knoppix uses vanilla kernel with only 2 exceptions:
Q: Are there any special components/patches in the kernel used on KNOPPIX?
A: No. KNOPPIX uses the currentmost STABLE vanilla Linux kernel from
www.kernel.org, with 2 exceptions:
- Until the XFS filesystem is officially accepted in the stable kernel
releases, the xfs patch from www.xfs.org is added
- The boot commandline is extended to support 512 chars, the patch for
this can be found in /usr/src/knoppix-kernel.patch