As you do your research, think broadly and creatively about where your target fits. Some industries are too new or too odd to fit into an established category. It also doesn't hurt to look at more than one industry.
Use industry profiles for a broad overview of the industry including the Five Forces
The Five Forces model was created by Michael E. Porter in 1979 to describe how competitive forces impact an industry. The five forces are threat of entry, bargaining power of buyers, threats of substitutes, bargaining power of suppliers, and industry rivalry.
Once you have identified your target markets, you can use secondary research to determine the size of the segment and whether it's growing, shrinking or stable. Market Research Reports do a good job of outlining the macro trends of these target markets.