Good inputs thus far. Here are some things to consider. NDA - unfortunately, it's very immature.. We build products for entrepreneurs and it's true that our clients ask us to sign NDAs as well.. but in reality it should be the opposite .. you should talk to as many people as possible.. It actually helps more than it hurts.. Ultimately, ideas are dime a dozen - execution trumps it.
Next - are you someone that can sell on a powerpoint? If so, go sell first, get the first letter of intent and then build the product. If you can't get someone to commit to buying based on your powerpoint - you need to build the MVP.
Finally - from what I read, it seems like you want a co-founder with sweat equity (I might be wrong because you mentioned "basic output like a SOW or pricing".). You can find such developers at the usual places (e.g. hackathons, meetups etc but not freelancer sites) but one thing that you do need to re-consider is the level of involvement that this co-founder can provide you (for free).. Typically, the developer needs to pay his/her bills as well (no matter how awesome your idea is - they still need to eat), so they will not be able to dedicate a whole lot of time into this..
As you are well aware of, time to market and idea validation/finding product-market fit is most important.. Are you willing to wait for 7-12 months for your co-founder to find time to develop the product before you can go to market? Could you hire someone to get to market faster? Would getting to market faster benefit you? These are all questions that only you can answer and come to a decision yourself..
if you have funds available, I don't know why software development firms would not be willing to take on your product development.. If you share more about how you were trying to engage, maybe I can shed more light.
All the best Jim.