Be prepared a lot of people will not sign an NDA.
Reasons include:

- they have multiple interests, and may already have something similar, or may identify someone who can execute in the future; an NDA prevents them from pursuing this in the future or possibly moving ahead on current developments

- you need them more than they need you; you have no leverage, that's why you're asking for their help.

It sounds like you are not looking for a technical expert, you're looking for a business expert.
Consider:
- customers are your experts that provide feedback on feasibility;
have you launched a minimum viable product to obtain feedback through?

I highly recommend learning lean startup methods and evaluating whether they offer a better route. They will help you learn regardless of who you discuss with, and
it may be easier & faster to engage experts *who will USE the product
than those who will have an opinion on the product.