Well first of all let me point out that you can also craft your own runes, if you read the runecrafting guide. Second of all, South of falador you will find port sarim, Talk to one of the sailors there and they will transport you to Karamja for a fee of 30 gp. As you exit the boat walk down the dock plank thingy-ma-bob and you shall see a little hut, behind that is the air rune spawn. Edit: The air rune spawns will take a while and wont offer many runes so id definately recommend the runecraft option.