I think that you should use the Google Keyword Tool suggested above and then break down your Niches into specific categories.
So if you were advertising "Baby Products" then break it down a bit to a specific brand like "Fisher Price", then break it down again into a specific "Fisher Price" product, like the "Fisher Price Papasan Cradle Swing".
So
Baby Product > Fisher Price > Fisher Price Papasan Cradle Swing
The Exact Term "Fisher Price Papasan Cradle Swing" gets 40,000 searches a month. And the competition is pretty much close to None.
So what I would do next is actually purchase the Domain Name "FisherPricePapasanCradleSwing.com" and build a review site around it like a price comparison, Video reviews, Written reviews, and a good SEO article or two.
Here's a site I did just a couple of days ago you could use as an example:
http://fisherpricepapasancradleswing.com
Once all that is complete then just start building some links "white hat" by submitting your sites to Social Bookmarking sites, and Directories. Then you should think about purchasing some text links on relevant sites. The more relevant links to your site then the better. And if you can get any legitimate .edu and .gov links, that's even better.
If you want to start advertising actually merchandise, it might be a little easier to start off with then Clickbank. You can sign up at
http://cj.com or
http://linkshare.com and start promoting for major companies like Toys R Us, Wal Mart, Target, FYI, pretty much any store you wanted to.
The payout isn't that much but you can find big niches with little competition with no problem at all, so SEO would be sweet for this.
Sorry for the long post, but I hope this helps a bit.