Do electronics distributors require NDA or manufacturer consent to buy certain items such as imaging sensors?
Assuming that you have told us everything about your situation, NDA's and manufacturer's wishes have nothing to do with this.
The manufacturer's won't care if you go through a distributor and buy their products. In fact, it seems that they prefer you to.
The reason for this is quite simple, and they've already told you it. They don't want to engage with you as you are looking at a low volume of sensors. They consider it as unprofitable dealing with you due to that. This is exactly why Distributors exist.