The only place you can farm Dreamleaf is at Val’sharah. It seems that this herb usually grows around flat grassy plains. So you should be able to find them as you walk along the foot paths. When you are collecting your Dreamleaf herb, there is a chance of a Nightmare Creeper spawning that will drop even more Dreamleaf. Just walk over them to collect them.

Here is the map to farm Dream leaf in Val’sharah.

Map of Where to farm Dreamleaf in Val'Sharah

Hope this help you with farming Dreamleaf in the World of Warcraft!