DMOS, Ebay and Amazon.com are good places to look for niche markets. You just have to look at the directories and dig in. You can also see what is selling and for what price. Follow the instructions in the tutorials to do this or just start digging.

Another great source for niche markets is ubersuggest.org. You will find everything from A to Z.

Another way to use a good keyword tool like JAAXY where you will find all kinds of new markets that you can pursue.