You sure here about keyword planner that server by Google Adwords to us, but more keyword tools are available on internet with the free of cost. Just need to analysis.
Twitter - find out trendy words from past present basis and fix your keyword. You can take help from # symbol. This is the best for you choose long tail keywords.
Facebook - It is another way to find out keyword same strategy as like twitter to find keywords.
Quora - Analysis what people are searching and what are right for that.
Google+ - Use #tag
Also at the bottom side google suggest some similar queries.