Hi Roberto
the glass is hard to find second hand and very expensive new. It just so happens that there is some on ebay at the moment. Buy new window rubber from Moss rather than go second hand. Just a note, if you are replacing the headliner that needs to go in first then the windows. The liner wraps around the window channel both front and rear. and around the top of the doors. VB has the glass but it is close to $2000 for the 3 piece set. It is also available from the MG owners club in the UK I think.
Moss part # 280-940 front seal
Moss part # 280-950 rear seal
http://cgi.ebay.com/ebaymotors/MGA-Coupe-Rear-Glass-Driver-Side-Section_W0QQitemZ160329887927QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item160329887927&_trksid=p4506.c0.m245&_trkparms=72%3A1171|65%3A12|39%3A1|240%3A1318
Mark
1958 MGA coupe ( under restoration
1972 MGB GT
1995 Ford Bronco
2004 Pontiac GTO