Where do you get rune without buying it. there are two ways that many players are able to get rune... assuming you meen the rune armour set or rune wepons and not magic runes.
1. one way to get rune without buying it is getting it as a drop by killing a npc (monster) or other people in pvp worlds..(one easy way to kill other players in pvp worlds and not use rune is stand at the g.e. in monks robes and fite sommone no armour and hope for a rune drop.) it is said that cockroach soldiers drop rune sq shields and rune scimitars as a rare drop.
2. another way to get runeite armour or weponds is by having the smithing skill to make the armour yourself.
the best was to get full rune is to buy it .. i suggest peice by peice... everyday just buy one peice and by the end of one school week or work week you'll have full rune hope i helped