It bums me out to see people say that they are having trouble with their world or campaign building because they can't locate just the right monster, deity, treasure, etc... in one of the books, magazines, etc... that they have.
Here's how I look at AD&D. There are 3 main things we find in the three core books, the Players Handbook, the Dungeon Master's Guide and the Monster Manual 1.
1 Game rules. These are the most basic rules on how to keep the game going and how to help you, the DM, make rulings. How to play the game in general. How to determine the results of an interaction. How is something "supposed" to behave?
2 Game mechanics. These are the details of how to make in game determinations. For example, how do you determine who goes first in a confrontation? The answer to that is a game mechanic. What is the process of conducting combat? Again, that is a game mechanic the "how to do it" information. Which dice do we roll to determine a score or result, those kinds of things.
3 Details and content. The books start off a beginning DM and Player with some ready to use stuff they can add to their games right away to get started. This content is not the last word or the only things that we can use, just something that can help keep the game common enough and easy enough to get anyone reading them off to a start.
Just because something we want is not in a book or adventure or published somewhere doesn't mean a thing. The best thing about AD&D is that it is YOUR game and it is MY game. AD&D is not meant to be a cookie cutter game system where the same things are in place each and every time by each and every person to play it.
We can create our own creatures, magic spells, weapons, enchanteed items. Anything at all that we can think of and give stats to. This is one of the reasons I like OSRIC so much is that because the published books are trademarked and tied to money, I can write my creations up as OSRIC compliant and share them with others without fear of getting sued or harassed.
Regardless though of whether we share our creations with others or not, the fact remains that we are nearly unlimited in what we can put into our games and still have those games be recognizable AD&D.