All of this is quite a long shot. It's a long shot, but remotely possible. You know, if no one has purchased the film rights, they just might be available at a moderate cost. Show a sample of your work, and make an offer to the author or publisher. If they have no others, they may give you a one year option for quite a low price.
Spec writing is already a gamble.
See if you can get the rights. If not, make an informed decision. If you love the material so much, maybe you should go ahead. You just might beat the odds. Maybe your love of the material will show through and enthuse others.