Looking for extremely long stiff fur.

I've never come across faux fur like that, but you might look into Taxidermy sites that offer faux fur, because you might run across something of the correct stiffness there.

Without knowing why you would need such a fur it is hard to recommend alternative materials or techniques you could use. When building a suit you have to compromise a lot with nature. It is difficult to get results that mimic nature if only for the reason of relaive size. Longer fur has to be much stiffer to behave the same as shorter fur and this stiffness limits its marketablity and therefore its manufacture. A long soft fur that behaved the same as shorter furs is the real challenge.

I believe that if such a product were available everyone would be using it for fox tails and the like, rather than the pillow-like contraptions currently worn. The trouble is when taking an animal at its original size and increasing it to "anthro"-size, the diameter of the individual hair-shafts of the fur must increase by the cube of the of the proportional increase in height.

For example, consider a fox two feet long from the top of its head the the soles of its hind feet. If you were to build a suit that is six foot tall from the top of its head to the soles of its feet, (three times as tall), the fur kept in the same scale would have a shaft 3 times thicker and three times longer as well. However, each individual hair shaft would be 27 times as heavy. (Weight being a function of volume - a 2" cube has a volume of 8 cubic inches vs a 6" cube with a volume of 216 cubic inches.) The fur would hang very limply from the suit. To get the same "bounce" or relative strength from each hair shaft the fur would have to be 27 times as thick as the original animal.

Such a fur would be too coarse to be much fun and too heavy to wear.