The source code still seems to be available at the original link
Grim Mouse is, as it says in the title, a fan made mod. I'm not sure whether this got merged with ResidualVM's main branch, I don't think it ever did although the developers themselves would know more.
The man who developed this (tobiaspfaff) might not have time to maintain it, or synchronize its changes with ResidualVM's main branch.
Unless he decides to reupload it himself, or someone else has it on their computer and can provide links, OR someone else builds it and takes it upon himself/herself to upload it or host it somewhere, your best bet would be to build it for yourself.
Having said that, building it shouldn't be too complicated even without much technical know-how, especially if you're using Linux/Mac. It isn't that complicated for Windows either, although there's a bit more fiddling about with the libraries, or you can set up MinGW as described on the build instruction page linked below.
If you pair what he said here:
...along with the instructions here:
http://wiki.residualvm.org/index.php/Bu ... ResidualVM
...you should end up with a functioning version of Grim Mouse.