The source for Kodi can be found at https://github.com/xbmc/xbmc/ - There are tags for all previous releases. 
(The source for xbmc, the previous name of Kodi, up to version 13 can also be found at this address.)

After cloning the repo you can list available tags with 
# git tag -l
and then checkout the release you want to compile with 
# git checkout tags/<tag_name> -b <branch_name>
for instance "git checkout tags/16.0-Jarvis -b 16_0" will checkout the 16.0 tag and name the branch "16_0"

It is also possible to directly download the source for a specific release from the github api like this:
# curl -L https://api.github.com/repos/xbmc/xbmc/tarball/tag > tag.tar.gz
For example "curl -L https://api.github.com/repos/xbmc/xbmc/tarball/16.0-Jarvis > 16.0-Jarvis.tar.gz" will download the 16.0 source into a tar.gz file called "16.0-Jarvis.tar.gz"