The GitHub repository has all the source code for MonoGame. Git's version control allows easy collaboration and backup, allowing you to select previous versions if need be. More information on that here.
You can find the older builds on TeamCity, but I don't think they keep older installers around for long, if at all. It looks like this version is the one you're looking for. Using the information on the page, I traced it down to this commit.
So if you had the repository cloned and are familiar with version control, you would simply checkout the revision
307685cf0b2b736bfbf39b677b5f2042e167dfb2 then build from source to get the DLLs you need.
Though I was already familiar with GitHub, I wasn't keen on building from source until I encountered issues with certain versions and couldn't run installers for previous versions since they didn't exist anymore. Now I actually find it better, since building includes the XML comments, which are curiously absent from the installers, for Visual Studio's Intellisense.