Please ONLY list places with level 67-70 "casters", since they are easier to farm with a passive pet.
No point in farming yellow mobs when you can farm green mobs which are by far much easier.
Also, you may want to look here for a bit more help. It was a previous thread started about the same question. But otherwise, your best option would be what Aethien said. They are easily solable and they never go into melee range.
I used the ogre casters on Warmaul Hill in northwestern Nagrand to level up my lowbie pets. You get beads and dust drops from them too. The beads can be turned in for Kurenai or Consortium rep. The dust for Halaa tokens. That location does have a few melee ogres too. I worked on my chain trapping and kiting with those. ;)
You could try to get a few 70 friends and run regular ramps. You can even have your pet try to dps, though I doubt it'll get any hits in.
You can also try the elementals that the sporregar guys fight in Zangarmarsh, it's by Umbrafen Village(I think that's the name). You can also get rep with sporregar that way.
Ramps is gray to you if you are 70. My suggestion would be Underbog. If your MM just do a SH run. You won't notice a big difference and you can still be on add duty for the final boss =D
Perhaps it is just me but i find castors do as much if not more damage than melee types ?
i prefer farming the warp stalkers and then the casters when my pet is starting to do a little damage , Main reason i hate the castors is they are always being farmed and i always get them spawn on me ,4 casters doing damage is not fun to me