I've found that the spiders in Orr drop these. Haven't found a specific location that's good for farming them yet.
Instead of directly farming these another great way to get them is by opening the "Moldy Bags" in the game. These have an extremely good chance of giving you a fine crafting material (a blue one, like this one) and are rather cheap on the Trading Post. Usually ranging from 20 - 50c a bag, depending on the type of bag.
While you won't get rich buying bags, opening them and selling the reagents on the Trading Post you definitely will be able to stock up your collectibles tab at an insanely fast rate. I made a video on how this method works, if you'd like to see it: Fine Crafting Material Farming Video for all skill ranges.
Here are a list of all the bags in the game so you know which type you'll need to buy for whatever item you're after.
Tiny Moldy Bags (0 - 50 Components)
Small Moldy Bags (25 - 100 Components)
Light Moldy Bags (75 - 175 Components)
Moldy Sacks (150 - 250 Components)
Medium Moldy Bags (175 - 300 Components)
Large Moldy Bags (300 - 325 Components)
Heavy Moldy Bags (325 - 400 Components)