Where is the air rune crafting altar?

That's between Varrock and the Barbarian Village - a bit south from the path.

You can find the location of any altar in the RuneScape manual - look for Skills, then Runecrafting. There are maps for each of the altars. You can also use a Talisman (in this case, an Air Talisman), that have an option to tell you the direction at which the altar is located.