Best thing to do is only submit to directories that make sense.
Google do not like paid links or paid directory placements as it undermines the way search engines should work.
I would recommend investing the money and time on the content of your site and let people link to you which will happen.
If you go to the
business card design page of my web site, (I do not want to create a hammock page) you will find a link to an excel spreadsheet that lists 30 or 40 key phrases for hammocks in the top right hand side.
I am in the process of making a heap of demo keyphrase downloads for common vertical markets that may even be available tommorow (thats what I said a week ago....) that will list around 150 keyphrases for each one.
I am guessing I will get approximately 1 request for hammocks so I will not be doing a full keyword research on it unless someone pays me, but this list should get you where you want to go.