Use this free tool provided by google:
and this one, KwBrowse.com - A Keyword Map For the Whole Internet
Did you try http://www.Hittail.com
I personally use it and it gives good suggestions.
Why use for the paid one when there is a free one
Google keyword external tool is give the good result
Or if you wanna detail, you can try MNF (micro niche finder)
In addition to the tools to help you find the long tails, I recommend the following
1) Look at you site logs and see how people are fining your site. You may find a spike in traffic for some longtails and start to target them.
2) Content is king - Every day there are 200,000 to 300,000 longtails used that have never been used before. How do you capture this longtail tarffic. Write articles natural and seed them with the two and three word short tails. You will then have hundreds if not thousands of longtail combinations depending on how many articles you post.
I find keywords from ebay pulse and google trends..also a li'l bit of imagination..
How to find keywords? How about starting with your targeted audience and understanding what they might be looking for?
Subscribe to news feeds, and write articles about hot keywords based on the news and what is happening today.
One way that I have found good keywords is to site someone down in front of the PC and tell them to search. It gave me some insight on how people that aren't familiar with the niche would search.
Kind of like an informal marketing panel.