A great aunt died of alcoholic dementia at 999 Queen Street,Toronto. How could I find the address of her husband when she died and where she was buried in the hope of finding what became of him.

Records from the Queen Street Mental Health Centre are held at the Archives of Ontario. Depending upon the year your great aunt died, you may be able to access those records (more info). As for her husband, unless you know the city/town where he/they lived there isn't much you can do if they were living past the 1901 census (if living prior to the 1901 census check census records). If you know the city check their local directories or telephone books.

I don't know how to go about finding relatives that can supply me with general info history, medical.

Start with who you do know and request names & addresses of other relatives. When you contact these relatives ask the same question. Before you know it you'll have an address book full of relations. Another tactic is gleaning information from family documents such as obituaries, birth notices and the like. They usually provide names and in the case of some obituaries locations (ie, daughter Jane from Toronto, ON). Then take this information and scour the phone books for any matches.

I am searching for my g-g-aunt and think I found her in the 1871 census listed at Rockwood Lunatic Asylum. Is there anyplace I can write to find out if it is her and any info on her?

If this is a provincial hospital, the records are held in the Ontario Archives on Grenville St., Toronto. If it is a private facility, then I believe the records were not required to be kept much beyond 20 years, if that.