I used to farm these in Eastern Frontier.
Start at Frontier Gate and follow the path round toward Ruins of Surmia.
Kill every grawl along the way, then map back and start over.
If you bring a good IMS, each run is about two minutes or less.
Collect all the Grawl Necklaces and salvage them, they always turn into Plant Fibers.
I got the amount I needed pretty quickly, although...
there are probably much faster ways.
I farm FoW, and salvage the Gloom Seeds that drop. Gloom Seeds salvage to ~15 plant fibers each. Probably better spots for plant fibers, but I don't go to FoW just for that.
Another trick is to pick up collectables that can be traded for monk Istani armor pieces at the collector in Lahtenda Bog. These can be salvaged for fiber. (Of course, you will need to have a monk character who can do the trade, though you can use another character to farm the collectables.)
Sentient roots, sentient seeds, and iboga petals are easy enough to farm around Plains of Jarin, and salvage into fiber.
Also, all varieties of bramble bow can salvage into fiber. You can farm Pongmei valley for these, plus dragon roots that salvage to fiber, and the chance of direct fiber drops.
If you can solo the big mobs of plants in Holdings of Chokkin, you have a good chance at picking up fiber and fiber salvage items.
Even clearing the map with a full party (it's a pretty safe area to farm XP for survivors), I'm getting a stack every couple of days, and that w/o even picking up and salvaging the sentient seeds.
I farmed plant fibres for an hour and managed to get 75. I farmed the dragon moss near Saint Anjeka's Shrine. The pvx build and how to use it are here: http://pvx.wikia.com/wiki/Build:E/Me_Searing_Farmer
You need the "A New Escort" quest for the dragon moss to appear.
Edit. To increase my chances i flagged the heroes and henchmen next to the portal. Make sure they are not on your radius when killing the dragon moss
I found quickest way and very lucrative was to farm those thornbush(spelling) in nightfall, where the last z bounty(Wahlli) and just out of sunspear sanctuary, if you do it in HM, they drop random drops of 24-28 and then salvage their plant parts they drop(forgot name lol).
Sorry for being vague but most people will understand where.
Farming Grawl outside Ruins of Surmia in Normal mode is a piece of cake. Grawl necklaces drop very frequently and be salvaged into plant fibres. Sure you need to farm about 75 of them, but its quick and brainless
dragon moss , they directly drop plant fibres at times, their dragon roots salvage to plant fibres, and bramble items they drop salvage to plant fibres, i got 2 stack in 2 hours + a stack of bday cakes + rares using 55 soj
** TIP ** make sure u stay outside of spellcast range or they will heal with aura restoration , also kite a few of their attacks with soj up so they dont all die on mass at the same time, your drops will be better