Just use your own understanding of the subject. That's what LSI is really about, and what it is moving towards:
"payday loans"
"payday loans online"
"fast payday loans"
are practically the same keyword. Those are not LSI variations.
Semantic as defined in the Free Online Dictionary:
1. Of or relating to meaning, especially meaning in language.
2. Of, relating to, or according to the science of semantics.
So what are the terms related in meaning to "payday loans"
"borrowing money"
"financing purchases"
"income advance"
etc... those are your True LSI variations. Simply repeating "payday loans" surrounded with other adjectives will still get your anchor text optmization way too high for "payday loans"
The LSI terms will 1) boost authority to your site with either content about these terms present on your site, or and even better along with backlink anchored with those terms...
Now you have a site that may be targeting "payday loans", but is also providing relevant information on "semantically" (get it) related topics, as reflected by the content, and the backlinks coming into the site.
The google keyword tool will give some ideas, but it's also a matter of cognitively sifting through and finding what's appropriate... If you do it properly, that's just good SEO. You're building content on useful relevant information, not over-optimizing, but still keeping everything nicely focused on your target keywords. You're giving google exactly what it wants: information rich sites with lots of "semantically" related content for the user.... not thin keyword stuffed made to game rank, over optimized properties.