Every niche has either few or lots of terms people use to search and find sites, so where can one find these terms?
A good way to do this is to become the customer you want to sell to. Enter in the keywords you would use as a customer to search for what you wanted and use those. You can also use Amazon reviews and forums to see what people are looking for and what questions (demands) need answering. It takes a bit of time, but is worth it. Also, enter in your keyword(s) into the search engines and see what results come up in the first few listings. Look around and see what other people are doing, what they are answering, what solutions they are advertising (free or otherwise). Also, go to websites on your topic and search FAQs. They can be a goldmine of keywords.