Just look in your cPanel hosting account If you look at the statistics for the domain/site you are looking to get long tails for you will find a box called "Search Keyphrases (Top 10)" Now generally the top ten results will be the keywords you have targeted for your site or landing page. However if you click on the "Full List" link just under the header for that box you will see a good amount of other keywords and phrases used in search engines to find your site. Pick through this list and pull out the long tails. Hope this information helps...good luck!
Or you can just add google analytics to your site and within a week or two you will have a list of all the keywords that people are using to find your site in the search engines. Google analytics also tells you how many times people have use the keywords to find your site. Just my 0.2 cents Mega