You can purchase the runes needed for level 66 to 68 spells in the bazaar. The prices have come down somewhat, but you're still looking at 10 to 20k each, depending on the level (for some reason level 66 and 67 runes seem more expensive than 68 on 7th Hammer, not sure if this is the case on other servers). You can also try getting groups to fight the 1-group named bosses for runes. Noble's Causeway, Wall of Slaughter and Muramite Proving Grounds are the most popular, in order of dificulty and level of runes you are most likely to have a chance at.
For level 69 and 70 runes, you'll need to group in Muramite Proving Grounds or Riftseeker's Sanctum. If you have some friends who raid a bit, you might try doing the group trials in MPG. The ones I've done have had a level 69 or 70 rune over half the time, though others report the drop rate to be lower for them.
You turn in the runes to a NPC in Dranik's Scar, not too far from the point where the Priests of Discord will port you into the zone. With Invis, you can easily reach him and turn in any runes you've won or purchased. The spells are rewarded in a set order for each type of rune (Minor Muramite Runes for level 66, Lesser for 67, Muramite Runes for 68, Greater for 69 and Glowing for level 70 spells).
You can also win runes in the new expansion that can be turned in for Omens spells. I haven't spent much time in DoD other than on raids, so I can't really tell you as much about where you can win them and who to turn them in to. I'm sure there are others who can fill in more info on those.