lumby swamp
crafting runes at alters. Use runes ess at certain alter to craft those runes. Eg water alter you crft water runes.
You can craft air runes just south of the Grand Exchange in Varrock
For every 1 rune essence you craft into a water rune(s) you get 6 experience. You don't get more experience if you craft multiple runes for that one rune essence, just 6.
There is no special soul altar. From runescape.wikia.com: "The Ourania rune altar allows runecrafters to craft a random selection of runes, including the soul rune." So, if you go to the Ourania altar, you will craft random runes, including soul runes now and then.
You craft them into runes.
buy them from ge you dooush
At the air altar in Varrock.
The bestest and fastest way is to Craft Nature Runes.
The best way to get 24M is to craft nature runes.
Get rune essence and craft them at the air altar.
Water and Air
Buy Them at the grand exchange, craft them, or buy them from the mage guild.