Why not try Google search combined with some creativity?
Use search terms specific for your niche to understand what it is all about, then use additional keywords as filters to narrow down the search to the results about the business volume, e.g. "marker report", "sales plan", "business plan", "trends", "statistics", "volume of trade" etc.
There is no universal recipe, just dig into your target niche, read some forums, articles, blogs, reviews, company profiles and sales pages, and you will be surprised how your future clients will teach you the specific keywords to watch and much more.
Just love to read and comprehend, otherwise you shall consider to ask somebody else doing the search for you.
If you need numbers, then please be more specific and write on the forum about your niche ideas (e.g. "how to cope with pain", "overcome shyness", "how to become a public speaker", "how to build your own bio-gas generator", "how to save on music purchases", "how to shot great photos", "finding a reliable attorney", "DIY accounting", "earn a fortune by repairing antique furniture", "make a living as freelancer", "your dream vacation", "better sex for aged", "self defense for housewives", "learn Spanish", "sell your house at double price", "skateboard tricks", "cook delicious food to keep your husband home", "be a respected parent", "using Google as an Internet guru", "antique weapons", "get rid of body odor", "Siamese cats", "huge and fascinating DIY aquarium", "using webcam to keep in touch with your family and friends while you are working abroad", "razor sharp knives", "flower gardening in L.A.", "beautiful roses", "amaze your friends with magic tricks", "vintage cars", "don't be scammed again" etc.