The Glyphed Runes drop off named or high level mobs in lower tier pop zones, but are can drop off trash mobs in places like Fire. Best way to cheaply get them is to have a group that is EP3 or Fire flagged (or higher) and head there.
Glyphed Runes will get you a random level 65 PoP spell.
You have a better chance of finding the spell you want/need in the bazaar though (runes are no-trade, spells are tradeable) and some can be a bit pricey.