Re: Reading
Double,
Two things:
1. The term of copyright has been expanded, so the 1930's isn't going to cut it anymore. (And actually, the old rule was 75 years, so you were needing twenties anyway.) Virtually anything written in the twentieth century is off limits.
2. If I understand you correctly, you're not looking for a specific book that you want to/have adapted; you have a script and want to retroactively base it on something. If that's true, in my opinion, you are putting the cart before the horse. That way lies failure.
I don't think a public domain story exists which is servicably similar to your story. If there is one, it's not well known, so you're not likely to find it. You're searching for a needle in a haystack. You'd probably have to settle for a story with vaguely similar themes.
The story logic, symbolism, and characters are unlikely to mix well. You'll probably end up defying your story to meet the adaptation. Things will get convoluted, because you're trying to serve two masters.
You've got a good concept. It needs more development, yes, but you can do that on your own. You don't need the crutch of someone else's work. You're a new writer and need to develop your craft anyway. Explore your story. Write a zillion drafts. Write a five-hundred-page opus and pare it down to a hundred. Play with it.
It's your story. Make it entirely yours.
And for the record, almost no one adapts for spec scripts unless the source is their own work or a classic.
Good luck!