65 spells are from pop and god, I figure you mean 66+ though.
66-70 are from omens of war zones and from depths of darkhollows zones. They drop off nameds in those zones. They are muramite and shadowspine runes respectively.
66 Minor Muramite Rune / Minor Shadowspine Rune
67 Lesser Muramite Rune / Lesser Shadowspine Rune
68 Muramite Rune / Shadowspine Rune
69 Greater Muramite Rune / Greater Shadowspine Rune
70 Glowing Muramite Rune / Glowing Shadowspine Rune
70 (Song: Ancient Call of Power) Ancient Muramite Rune / Ancient Shadowspine Rune
Muramite Runes are turned in to Karsor the Mad in Dranik Scar, the entry zone of Omens of War. He's pretty much to the south of the zone in next to a tree.
Shadowspine Runes go to Brother Hidden Path in Undershore, in the southwest corner of the map, just outside a big Shiliskin tent.
You get songs in a specific order, as long as you have the songs you've already gotten either memmed or in your inventory (maybe your bank too), you will be given the next song in the preset order. Muramite and Shadowspine runes share the same sets, and are interchangable in regards to turn in order, but must go to their appropriate npc's. You can turn in and receive songs before you can mem them and save them for later if you like.
In addition to those songs, there are some other 68-70 songs. In depths of darkhollow, there are two spell mission arcs that yield two songs. A viral mez, creeping dreams, is the reward for the 68 spell arc (5 missions, starts with Kelliad in Stoneroot Falls) and a melee disc called thousand blades is the reward for the 69 spell arc (4 missions, starts with......Brovil I think, maybe Treddlehoop, in Corathus Creep). The viral mez is situational and only works on mobs up to level 73. The melee disc is exceptional, and makes you score very high melee crits for one minute.
In Prophecy of Ro there is an aura called aura of the muse (must have) that is rewarded when you complete the third skylance mission, the quest npc is in arcstone near the relic zone, someone the fourth born. Ao the Fourth Born maybe. There is a song called arcane aria that is the reward of one of a number of quests in the devastation. The quest npc is the first mob just past the destructible wall in Sverag, also known as Rage. You can chant kite trash mobs in devastation for the drops, any quest at all completed will yield the song. It adds a second 200 dd nuke to dd spells casted by group members, and is crittable.
71-75 it switches to a rank system. In the serpent spine, all songs are available for purchase from a minhoten vendor in the near western part of Gor`Karu Mesa. These are rank 1's. Rank 2's, upgrades to their lesser versions, are dropped by mobs in various zones. Jonthan's (71) is purchased in the buried sea or katta castrum with currency rewarded from missions, similar to don vendors. The catch is you need faction as well as currency, which can be received through those same missions, or solo tasks throughout the zone. Jonthan's is a self only melee buff. There's an upgrade to niv's harmonic there too, and...probably something I'm forgetting right now. Rank 3's come from raids.
76-80 are mostly vendor purchased for rank 1, some are dropped only. Rank 2's are acquired through factioning and vendor purchasing with appropriate faction, and some rank 2's are dropped. Rank 3's are gotten through raids.