In Winterspring Blizzard placed a merchant named Vi'el (loc: 59, 7
This merchant sells both Gromsblood(10 Silver) and Felcloth(80 Silver).
Respawn time I have found to vary however this is a very easy way to make money , just log in a few times a day to check to see if the item is up.
note: The Merchant is guarded by high lvl elites , however you can ether corpse run an alt to the cave or have a warlock summon your alt to it.
It is also preatty easy to bot while doing this , using an auto mouse clicker.I use a runescape auto miner(http://www.golddealing.com/mmo/). However there are probably better programs that you could use.