First, let me answer your top level question. If you are asking future distribution partners to sign non-binding letters of intent, you probably don't really need to offer any incentive. However, I would argue that non-binding letters of intent are not nearly as useful here as they might be in other situations.

If I ask a real estate agent or a small business owner to sign a non-binding LoI, chances are they are going to take it more seriously than the document actually is, which is what makes it useful for validation. However, when you are dealing with a larger company that signs distribution agreements on a more regular basis, they might view signing your LoI as being the same thing as telling you, "Yeah, this is a great idea. Of course I would buy them." even if they never would actually buy any.

I would suggest getting an LoI to put out some test placements in retail stores. It might even make sense to try and get some contracts signed, though my gut is telling me that might not be the best idea.

In the end, a non-binding LoI is already one of the lowest risks way to validate something. I don't think you should be offering long-term benefits to the other party unless they are putting up with some of the risk. Instead, focus on the benefits to them. You might say you will give your first 3 partners an innovation award or something to recognize their forward thinking status in their market and community, etc.