I made this Rain Poppy farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick this Herb from skill 1, but you will only get [Rain Poppy Petal] until you reach Herbalism skill 450-500 (10 makes one herb).
The Jade Forest
The Jade Forest is pretty much the best and only place to farm this herb. You can find Rain Poppy in Vale of Eternal Blossoms too, but there are so few of them, you will always have to wait for respawns. Rain Poppy can be found near water all over the zone, so you usually only have to look for places where there is water. For some reason it won't spawn on every part of the river, only on the parts I draw the route.
There is an alliance town at Pearl Lake, so horde players might not be able to pick every herb there, or you can just kill the elite guards, they are not hard to kill.