Wills were a lot more common in Quebec than in some of the other Canadian provinces. Unfortunately, our ancestors did not often make out wills unless they owned property or a business and quite frequently their land or personal articles had been distributed prior to their death. Wills were often not recorded till decades after the actual death.
If the will you are seeking is at the Registry Office the General Register (GR) is the most common book to use as an index (Also known as the County Book). Remember, however, not all wills are listed here. Any will describing land can also be listed in the Abstract Index. Many Register or Copy books are now on microfilm and have been distributed to provincial archives, universities, historical groups, etc.
Wills over 100 years old are in various regional archives and those newer than 100 years are in the offices of the notaries concerned and are not open to the public unless there is a need to settle a legal dispute. You might write to the local genealogical or historical society for help on wills.
Wills can also often be found filed by a notary at the local land office because so much of a will deals with inheritance of land.