Maintenance manuals are controlled documents that are revised on a regular basis. Most oudated mauals and CD ROMs are destroyed to prevent outdated procedures from being used and obsolete parts from being ordered and possibly intalled on the aircraft.
In addition, there are no general set of maintenance manuals for a particular aircraft, as they are custom tailored to the individual customer. A 767 built for airline x may have different equipment than a 767 built for airline y, and the manuals will reflect this.
I guess you have to know someone working for a particular air carrier to get ahold of old manuals, however if they (manuals) ended up in the wrong hands and contributed to an accident due to the wrong maintenance procedures being used, there would be hell to pay by all involved parties.