Hi there ga1000!
setserial is hosted by Sourceforge, which hosts many other open source
projects on the same site. You can see the setserial project page
The latest version of setserial is 2.17, and you can download the
source for this here (pick the location which is geographically the
nearest to you):
The most important file in the archive is setserial.c, which contains
the bulk of the code.
I hope this is what you're looking for. If anything's unclear, or the
links do not work as expected, then please request clarification
before rating this answer, and I'll be happy to help!
Thanks, and good luck
Already knew of Sourceforge, so I did a search for setserial on their