I'm having trouble importing FBX files from Daz with morphs, and a friend who is using an older version of Blender (2.78) actually has a newer version of the Autodesk Import/Export Plugin (and in the Blender docs I see there is a newer version still -- I'm using 3.1.17). How do I get a newer version? I've searched the web everywhere, but don't see any way to download this. Do I just reinstall Blender and assume the newest plugins will be there?
The fbx import/export addon is one that ships with blender, newer blender releases will include a newer version of this plugin.
When you say you are using 3.1.17, I think of two possibilities, either you are reading the version wrong or you have a non-official version of the plugin installed. Blender 2.79 includes v3.7.17 of the fbx addon. In this commit made Oct/2014 the version of the fbx addon went from 3.1.0 to 3.2.0 and that 3.2.0 version of the addon was part of blender 2.73.
In your preferences, find the fbx addon details and you can see the version as well as the path to the addon file. If the path does not match the installation location of your copy of blender it will most likely be your settings folder. If this does show a version of 3.1.17 and it is not part of your blender install, find and delete that addons file/s.
If you have somehow managed to get an old version of the addon to replace the official installed addon you need to re-install blender and ensure that the correct addon files gets installed.
If blender 2.78 which included v3.7.7 of the fbx addon is able to import files that the newer version doesn't, you should report that as a bug explaining that the older version handles specific files better.
Being open source, all the files used to create blender are publicly available, you can find and download any or all of them. The fbx addon can be found in blenders addon repository which is now at v3.10.0, this is expected to work with blender 2.79 and may or may not work fully with older blender versions.