If you have registered your 997 with Humminbird, then you can
get the newest update from there:http://www.humminbird.com/Support/
Version 5.840 looks like the newest update from the link on this site:http://forums.sideimagingsoft.com/index.php?action=downloads;cat=4
Some people here have used Universal Mania, and have been happy.
They are supposed to have the best prices.
I have no experience with them myself.