If your thinking of making up some mackeral feathers,flies for sea fly fishing,or want to tart up your trebles (tarting up trebles on even basic old style mackeral spinners has seen me catch Bass,if the Bass are there.)
Theres many places to get some materials.
One good source is Yorkshire Game Angling....have a look at the likes of,
Lureflash twinkle
Lureflash Mobile
Flouroflash
Maraglit
Mattabou
Holo g
I think the "Maxi Packs" are a good deal,you get a pack of approx ten colours in one go. Buy 3 different packs and get one free. Enough materials to last years. Good qaulity and good value. Plus most importantly it catches fish.