I find a partridge skin one of the most useful things I use for fly tying, along with a hare’s mask. The partridge feathers on the skin contain many shapes, colors and sizes of soft, webby feathers, and can be used for any number of wet and nymph patterns. It is not always necessary to follow the material listing of patterns exactly. For instance, partridge feathers can be substituted when a pattern calls for a wood duck tail. Just find the feather that matches the color closely. A partridge skin costs a few bucks, but it’s well worth the investment, and the skin can be used to tie many nymphs and wets for years to come.
You can buy smaller packs of other things, but in the long run, it’s more expensive. Here is a list of materials I believe are well worth stocking in bulk:
Hungarian Partridge Skin
Hares Mask – great amount of colors for dubbing rough nymphs with guard hairs. A $5 hares mask equals 10 $3 packs of nymph dubbing you buy, which are mostly cheap, dyed rabbit fur without the guard hairs needed to make your nymphs look gnarly. Also don't be a sucker and buy a pack of dubbing in every shade of color known to man. Buy some basic colors and blend them by hand or with a cheap coffee bean grinder to get the color you need. The same with tying thread - white thread + colored Sharpie = great savings.
Peacock Sword
Pheasant Tail
Marabou
CDC feathers - at least for me
Black saddle neck – for buggers
Dry fly necks in Brown, Med. Dun, Cream – a full or half neck of each is best, and Whiting/Hoffman capes are the bomb. The Whiting bronze grade is good enough, better than most of the other top grades. The capes have all the sizes you need, and you can buy 100 packs to supplement the sizes you use most when you run out. You can buy 100 packs in other colors when needed, but don’t forget, it’s a lot more expensive that way, especially if you tie flies in a lot different of sizes.
I’m sure there’s more others can add. Good luck.