If you google "Free competitor keyword finder tool" you will find many but you will hardly find any helpful in delivering reliable results...
You can go for SEMRush, Moz free trail to make this one-time task done.... Even the free tools don't show accurate results, their results may misguide you, all these tools work on a specific set of algorithms that don't claim accurate results...
Its better you go for a free trail or if a paid version is not of your much use, you can hire someone having a premium version of the reliable tools and pay them some dollars....