"How do you know that there will be merchants in your niche? ie: CJ wont let you have a look at their lists without registration and approval."
Don't need CJ almost every merchant in CJ has an affiliate info page on their site so just Google "green widgets" + "affiliate program" or even just green widget affiliate.
"If you are doing an advice how-to type niche...that pertains to a broad subject that is not really a product should you then look to your target audience in deciding and finding products to promote?"
Absolutely! If you know enough about the niche to do a how-to advice type site, then you should know how those people think. If you were one of the ideal readers, visitors or customers you hope to attract - what would you be interested in buying or learning more about? If it's advice site I assume there is a problem in there somewhere. Find out what the core problem people in your niche have is and then find products that offer a solution.
If you have no idea what they would be interested in or what products may be a fit there are 2 approaches you can take.
Go to site like
eHow | How To Do Just About Everything!, Yahoo answers and 43things and search to find out what problems people in that niche have, what they talk about, what they are interested in.
Go to Google or sites like ehow and type in your topic including how to.
How to do yoga or how to grow lilies or whatever. Now in addition to looking at the content being written about tune into the Adwords ads being shown on Google and the contextually relevant Adsense ads being show on ehow pages about your niche. Those ads are all people selling SOMETHING that should be fairly relevant to your niche and probably many of those products have affiliate programs.