Mongolian People’s Revolutionary Party Archive
Website: n/a
However, try Sergey Radchenko's own website http://mongolhistory.radchenko.net/ for samples of documents and http://www.radchenko.net/campaign.shtm for more information on the Mongolian archives.
Also, see Chris Kaplonski's website, http://www.chriskaplonski.com for samples of documents in the Mongolian script.