With World Dating Partners we often discover niches through partners by accident, this is a regular occurance, but through this experience we have discovered that niches are not so difficult to find with dating its slightly different as there is only one system that deals closely with this, but with other products slightly more difficult, or at least I don't know how many companies can do this with other products.
What I can do which although related to dating would actually apply to anything is to look at the different areas you need to look into to discover these. If ytou want to research this look into a combination of factors..
Product
Ethnic group
Country
culture
interest
health
To my mind the most important two of these is Ethnic and country
Most of these niches belong outside of the US look more towards countries that primarily speak English in Europe, parts of the Middle east. Excellent examples of these are the Netherlands, Denmark, Iceland, Austria, Belgium, Italy, as well as New Zealand and Australia.
If you can't find a company that doesn't specilise in niche sites, then try and approach other cmpanies working within you researched area. Most of these will be happy to supply you with data. maybe : )
Once you have dicovered you niche, produce a mini site to explore this or find a company that has an affiliate service within the range.
Exploring many niches in the same area are often an ideal way of dealing with this, so you can cross sell similar products or services very closely related, this way you can cross sell different services as well as products.
discovering a popular topic in terms of information is an ideal way to do this, as a dating system, we would o this by looking for a site related to a popular niche and then help this site by producing a relevant dating sites, then within that site sell products related to dating for exaample which again related to that niche.
There is a theory of fragmegration
Sample Chapter for Rosenau, J.N.: Distant Proximities: Dynamics beyond Globalization. which expores that the larger the size of any community the more it fragments into smaller communities. Hence ethnic groups who tend to group within other countries to form large niches based on ethnic origin or religious communities.
Sorry, seemed to have gone into over kill with your questions, where I basically don't have a clear answer for you except in my own world of dating.
Anyway I hope you found this useful regardless, if you do want to know of any specific niches let me know, dating is often a way to determine this and we ofteen hold quite complex data on worldwide niches.
In terms of producing them, we have lots of ideas if you find a particular niche let me know and I'll try and dig out some of our own research to help and know varyious ways of approaching htis sideways in terms of selling and marketing.