src.txz

I think you should be able to get it from the memstick installer image under /usr/freebsd-dist/. We used to be able to download past releases from FreeBSD.org's FTP Archive but the link isn't working right now.

As for your second question, I suspect not but don't really know.
 
If there is no src.txz file availlaible, you can get get the source for 11.1 from svnweb or clone, download as zip file from GitHub. Afterwards, if you wish, compress the source tree yourself to a src.txz file.
 
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