I guess it's difficult to find roommate without meeting the guy in person. although some people seeks english speaker for their roommate so that they could practice english at home.
but you didn't even tell how much is your budget. the place you want to live cost pretty much, you know. make sure which train line you want to live. To go to Aoyama Univ, 山手線Yamanote Line, 埼京線Saikyo-line, 田園都市線Denentoshi-Line, 東横線Toyoko-Line, 井の頭線Inogashira-Line, 千代田線Chiyoda Line are convenient.
there's some websites for foreigners in japan. try check these.
http://metropolis.co.jp/
http://www.seekjapan.jp/japanzine.php
http://www.tokyonoticeboard.co.jp/