Out-of-force Acts are not maintained in the active Register and are thus not available online via the Department of Justice or the various Canadian legal libraries.
As such, if you want to find the Petro-Canada Limited Act of 1975 (or the amended version of 1985), you're in for a jaunt across the border to the Library of Parliament where archives are kept.
Mind you, there is the remote chance you might find it on some obscure website via Google; but I don't see anything promising on the few four or five pages of a search I just did.
In the meantime, here is the
Petro-Canada Public Participation Act of 1991 which dissolved Petro-Canada Limited as a Crown Corporation. Absolutely not what you want, but whatever.
