Another useful source is to find online fiber exchange groups - even if you don't have fiber to exchange you can sometimes buy fiber at very reasonable rates!! There's one lady that sells me her skirting (cleaned) - usually takes a while before I can get it, but I get a lot very inexpensively! That particular fiber I can't use for most of my work, but for projects that need a fiber with a really fine crimp - her shetland fiber is perfect.
For core fiber, I buy wool batting from wool mills - it's meant to be used in comforters and quilts, but makes a wonderful core fiber.
4 oz. of wool costs me about $8 from Mielke's Fiber Arts. They've always had great prices and excellent quality! I've also bought fiber in bulk from a variety of places - usually "specialty" fiber rather than the Merino of which I use a lot. I'm trying to make arrangements when one of my friends in Europe to buy short fiber Merino batts for me as she can get that a lot cheaper than I can in the States. I have dyed fiber and personally prefer to stay away from doing too much of that.