Here is a list of most of Sierra's AGI games released in the 1980's:

Game Title	Release Date
The Black Cauldron	1986
Gold Rush	1989
King's Quest 1: The Quest for the Crown	1984
King's Quest 2: Romancing the Throne	1985
King's Quest 3: To Heir is Human	1986
King's Quest 4: The Perils of Rosella	1988
Leisure Suit Larry in the Land of the Lounge Lizards	1987
Manhunter New York	1988
Manhunter 2: San Francisco	1989
Mixed-Up Mother Goose	1987
Police Quest: In Pursuit of the Death Angel	1987
Space Quest 1: The Sarien Encounter	1986
Space Quest 2: Vohauls Revenge	1987

Many of these games are still available in several of Sierra's CD-ROM collections. These are the King's Quest, Space Quest, Leisure Suit Larry and Police Quest Collections. The rest of them, for the most part, are unavailable through retail. It is possible to find them through the Abandonware Webring. I don't condone pirating games, but then, if Sierra didn't want us to pirate their games, they should make them available.
 
There are other Sierra AGI games such as Donald's Duck Playground and Winnie the Pooh in the Hundred Acre Woods, among others. These seem to be almost completely unavailable now. It seems they were never even made available in a format todays computers could run without converters or emulators. A few web searches will shed a little more light on these "lost" games.