I have been trying to do the same. I have two niche markets I have identified. With one, it is sufficient to search the internet for similar products (do a search as if you were searching for your own product), go to the products web page, and simply take inventory of its features. Most of them have demos and and free trials. I take advantage of ther free trials and play with the applications to see what the interfaces look like and help me pinpoint strengths and weaknesses of my own.
The second niche market I have identified is not this easy. My biggest source of info is going to be users of similar software or those who would be targeted by these offerings. Interviews are the most common source of requirements gathering. I am actually going to have to set up interviews with would be users...stressing that I am simply gathering info for educational and informational purposes (people have less tolerance for sales pitches). Most likely, people who are using the software will have been approached by a variety of companies, and will have the spec sheets and/or bids from competitors in order to choose. Most of the time, they don't mind showing you, at least, the spec sheets and briefly discussing what they did or did not like about it.
You must identify your stakeholders (managent, IT personnel, finance managers, and users), and then use a variety of techniques to obtain your info if you can't simply find it on the web. I am lucky to still have my MBA text books that show me the specific techniques to use, but you can ook up "requirements gathering" techniques in order to make the best use of your interviews, holding JAD (Joint Application Design) sessions with stakeholders, designing questionnaires, and carefully analyzing the collected info.