To find the cheapest you'll have to shop at different stores. What I have noticed buying material online is that while one store may be cheaper on a certain item, you can't assume that they are cheapest for all material. With intruders having so many different styles and materials, it would be difficult to just recomend one store without even knowing your materials list. (ie do you plan on using rhea, ostrich, or both...do you plan to use artic fox, dubbing, flatbraid?)
Part of the fun, at least for me, is also the hunt. Give it a try.
Like stated, you will have to shop around for places that carry your materials and possibly one store may not have all you need. Also check craft stores, great place for materials as well as Dollar General, Dollar tree and such. Hobby and craft stores are also a good bet
When I can't get something local, I buy from J S Stockard on line. They have a quick turn around time and the prices are comparable to othe on line stores. They have stuff you can't find in local shops as well. jsflyfishing.com
best deals in terms of value, not necessarily price, are those which offer free shipping, either outright, or with a minimum purchase. I always figured i didnt mind spending an extra buck on a couple items if i was getting to use the money i'd pay for shipping at another store, on materials instead a store with slightly higher prices....
I use caddisflyshop.com for alot of items. When you spend $25 plus the shipping is free. They also deliver fast and their customer service is great. For tungsten beads I use riplipsfishing.com, they also sell on ebay and amazon. Hope this helps.