Agree with Eunan, you really need to shop around & compare. The sources he's listed have all been around many years. That's usually a good indication they're a reliable source, but doesn't mean they'll have exactly what you want or need.
I like the Hareline products too, but also will buy Wapsi & Raymond Rumpf brand products. ( All 3 are wholesale suppliers.)
I don't tie a lot of trout flies however, so my needs for materials are different.
Look around, do search's online & sign up for email notices & newsletters. Many online sources will have sales & closeouts on materials & they may not be widely publicized except via their email notices.
I'm signed up for emails & newsletters from JStockard, Orvis, Stillwater Fly Shop, South Holston Fly Shop, Pete's Fly Shop, Sierra Trading Post, Bob Marriotts, Allen Fly Fishing (great hooks), Theriault Flies, Fishwest Fly Shop & a few others I can't think of right now.
I also check out Ebay frequently, but you have to know what you want & what you're buying there to get good deals. It's best to ask a lot of questions of Ebay sellers if you're not sure!