Runes, where do i find them?

Hey, im fairly new to the game. I collect runes because i figured they have some value. my question is how do i find rare runes? ones higher then THUL? the only thing i found that i believe was a high leve was a pul rune which i traded for 3 items which seemed good to me. One was a shield including another high level rune, sigons guard, and another body armor. Is there something you could tell me which would imporve my search for high level runes?