I made this Grave Moss farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. Grave Moss is on of the rarest herbs, it can only be found near graveyards. It's recommended to have around Herbalism 105, so you won't get herb fragments.
Razorfen Downs - instance
This instance scales to level 30, so you can't solo it unless you have a character around level 35-40.
You can find the entrance to Razorfen Downs at Thousand Needles. (picture of the location)
As I mentioned earlier, these herbs only grow in graveyards, so it's really hard to farm without waiting for respawns because there aren't that many graveyards where it grows. Fortunately, you can find Grave Moss in Razorfen Downs and you can reset the instance to make the herbs respawn.
You have to kill most bosses in the instance because there are blocks at each boss preventing you from going further unless you kill the bosses. You can summon the spider boss if you hit that bell thing in her room.
There are usually 4 Grave Moss in the instance. 1 before the second boss, 2 at the second boss, and one at the last boss.
After you cleared the instance, go outside and reset it by right-clicking over your own portrait then click the "Reset all instance" button. This restarts the whole instance and all mobs/herbs will be back. However, you can only do 10 instance runs in an hour, this means if you are clearing this place under 6 minutes, you will have to wait a bit.
Hillsbrad, Arathi, Wetlands
If your level is not high enough to solo Razorfen Downs, you can try farming between these 3 graveyards in these 3 zones. If you have a fast flying mount, you will have to wait for respawns because it takes around 3 minutes to do a full lap in these 3 zones, but Grave Moss's respawn time is around 5-8 minutes.
Start from Hillsbrad, go to Arathi, then go to Wetlands and back to Hillsbrad. Or the other way around if you start from Wetlands.
Duskwood is also a good place, but you have to wait for respawns here too. (5-8 minutes)
Move between these small graveyards.