Hi
1. I can draft an NDA for you or you can find numerous examples online (although they are not tailored to your specific needs, they should be ok if you want to save the money).
2. Most mentors/experts/investors don't sign NDAs - we/they work with too many startups and have usually heard your idea numerous times before and therefore don't want to be limited. Second, it takes too much time (consider that they/we speak to tens of startups every day...if we had to spend 30 minutes reviewing NDAs we wouldn't get any work done).
3. In 99.9999% of the cases no one will steal your idea. Everyone has ideas, what makes entrepreneurs unique is that they actually implement them.
4. Sharing your idea with as many people as possible will ultimately help you (they will give you feedback, they will tell you if it exists, they will tell you what to improve, and they will criticize you and try pursued you not to do it - and if you listen, then it means you shouldn't be starting a startup anyway).
sorry for sounding 'tough' but all the above is the honest truth - and if you want to succeed ask for honesty, not praise.

I've successfully helped over 300 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david