You can buy it at ge or you can do the quest calles rune mysteries so you can actually craft runes.
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kill things like goblins or jsut buy it
It is called air altar. It is a little south of the road from Varrock to the Barbarian Village (i.e., west of Varrock). If you have trouble finding it, take an air talisman with you - it has an option to show you the direction. To craft runes, you need either an air talisman or an air tiara anyway.
If you make one(get one), it acts as a talisman of air. It is wearable. When you are wearing this item, you can simply right click on the air altar and select ENTER.
The air altar moved from its original location it is now located between the Barbarian Village and Varrock, use "Locate" option on your air talisman to get it's exact spot.
The Air altar may be found west of Varrock, east of the River Lum close to the bridge near Barbarian Village. To enter the Altar, you must either use an Air talisman on the mysterious ruins or click on them while wearing an Air tiara or an Air talisman staff. The quickest way to get to the altar is by breaking an Air altar teleport, obtained from the RC Guild.