Shocking how no one mentions a fine area to farm these.
It's true most, if not all mobs have an average of 20-25% chance to drop these...but I found out that the most annoying of Zangarmarsh creatures hold the key to gaining many Unidentified plants.
If you've ever done an instance in Zangarmasrh, you know of the lake to swim through has a plentiful amount of Mudfin frenzys' that every so often shoot up at you.
Turns out these buggers each have a 20-25% chance to drop an Unidentified Plant, and being they are all 1/4 of a regular mob, to AoE them down in a sense technically quadruples your chance of finding an Unidentified Plant.
I farmed these guys in bear form with no hassle and hit some mobs of 4 with no drops, to tanking a mob of 8 getting 7 drops...the odds are in your favor if you're out for the Grinding of Unidentified Plants.
Best of luck, and good fishing!