Google Keyword Planner Tool itself is great to find LSI keywords, but you can also use ubersuggest.org and lsikeywords.com tools that i instantly find through simple google search. Also you can include Google related search terms at the bottom of the SERP.
Tools will not bring back results unless you specifically search for them. For example, if you know nothing about soccer, you may not have known to input “football word cup” or “David Bechkham” into the Google Keyword Planner.
Google Search engine in best option to find LSI based keywords. But You should aware about "LSI Keywords".