Anathe is the last stage of fiber that is used in Tailoring. This is a valueable plant since it is used in making some of the best armor via Tailoring. Even if you're not a tailor you can make quite a bit of money from farming and selling Anathe Fiber. The best possible place for you to farm Anathe Fiber is in southern Beluslan.
The fiber can be found really all over southern Besluslan. The best possible place to farm it is right below Red Mane Cavern. You can find a ton of it near the Griffo's and Crookhorns/Darus. Right above where the map says "Frost Spirit Valley" is where I recommend you farm for Anathe Fiber. Of course if someone is here, you have many other places you can farm.
Another great place for you to farm Anathe Fiber is in Brusthonin. The Anathe Fiber in Brusthonin has level 46 - 47 mobs around it meaning you should be around that level before coming here to farm the Fiber. Luckily a lot of the mobs in this area are Neutral meaning they will not attack you unless you attack them first.