Generally speaking, old debs are stored in /var/cache/apt/archives.
But if you have run apt-get clean you might have deleted them. If that is your case, you have several options:
Try to purge mdm and reinstall it again (I guess you have already tried this...)
Edit your sources.list to point to Debian Wheezy repos (see below), and then run sudo apt-get update and sudo apt-get dist.upgrade. Fix the problem, and when the next UP is released move back to LMDE or to Jessie
You can try and install the package from [url=http://http://packages.linuxmint.com/list.php?release=Maya]the Linux Mint Maya main repo[/url] (I am not sure if it will work and remember it is never a good idea to mix packages from differente distros, so if you are gona do this, please back up your files, settings and packags with clonezilla)
You can always
report the bug and wait for the community to fix the problem.
To move to Debian wheezy, you need to edit your sources.list like this:
deb http://packages.linuxmint.com/ debian main upstream import backport romeo
deb http://ftp.debian.org/debian/ wheezy contrib non-free main
deb http://security.debian.org/ wheezy/updates main contrib non-free
deb http://ftp.debian.org/debian/ wheezy-updates contrib non-free main
deb http://www.deb-multimedia.org/ wheezy main non-free
# deb http://ftp.debian.org/debian/ wheezy-backports main contrib non-free
# deb http://mozilla.debian.net/ wheezy-backports iceweasel-release
Backports and iceweasel repos are optional (funnily enough Debian has just changed their policy as far as Iceweasel and Chromium are concerned and you can get recent versions -sid versions- of those packages in the security repo
)