When defining your customer, it's a good idea to first think about the companies that are your direct and indirect competition.
The direct competition are those business that offer services and products that are a direct substitute for what you offer. The indirect business are other services or producs that may fullfill the needs that your product or service fulfills. Subsitutes, as Michael Porter calls them in his 5 forces model.
Making a list of those potential competetors, is usefull for the next step, identifying the customers of those competitors.
The trick is to find as much information on what segment of customers each competitor targets, and why the client of this competitor buys for him.
Websites, productfolders or posting in front of the store may all be viable ways of getting a good idea who is the potential client of that competetor, and therefore your own.
Complete the competitors list with the kind of people each organization is targetting, and why that organization is successful or not. When you have done that, map all the types of people and find your niche or the weakness in the competitors' targetstrategy so you can take over their clients with a better proposition.