You'd be amazed how helpful some mail order shops are. My experience has been to avoid the big-name places and deal with the smaller owner/operator places where the person you talk to on the phone is "the"resident tier. You'll get nowhere with an order taker that knows only the catalog numbers. In dealing with the two or three places I frequent I can even specify details that I would look for in person like: "I want straight fiber calf body hair.", "I want long straight fibers on the calf tail for my smallest streamers.", " I want the deer hair tanned." etc.
In addition I always look for the brand names of materials in their printed catalogs. That gives me some assurance of consistancy. You should, at all times, avoid places that cut up and package their own natural materials. There is one unnamed popular shop I've dealt with that repackages any kind of natural material he can find with the quality varying between garbage and useless.
After tying for almost 60 years I'm still a happy camper. Good luck.