Just like Industry-level research, there is a wealth of information available for free online and within library subscription databases to help with market-level research, including data to help describe your customer segments. For a more detailed definition see Market Segment (via Investopedia).
In the course of researching your market, you will inevitably find 'the perfect report' that seems to match exactly what you need - but it will cost a prohibitive amount of money. While you have access to a number of sources for high-quality, professional, proprietary market research through the library, you will encounter reports that we don't have access to.
When doing market research, it's helpful to remember that what you are looking for may not exist (!) or it may exist and you can't access it. When this happens, you still have options like:
Being familiar with the general industry can help you make decisions about what proxy information is appropriate for you to use when describing your customers and calculating market size.
Identifying competitors can be a challenging but fun part of market research. Depending on the nature of your ideas, it may be more clear who you are working with, as is the case with direct-to-consumer ventures, or you may need to dig a little deeper into scholarly literature or patents, as is often the case for more technical ventures.
Here are some strategies for identifying competitors: