I'm afraid, with what I want to supply being in the fresh food realm that retail outlets may quickly create their own version of my product to cut me out of the equation.
Because it's in the fresh food realm, a patent won't do too much.
All I can think to do is offer them competitive prices when I supply my product to them which may lead them to abusing that and squashing my prices down unfairly.
It looks like you already know your answer: you can't.
Business comes with all kinds of risks. Someone cloning your idea is one of the risks. Distribution channel cutting you out is another risk.
You can't eliminate risk.
You can only accept the existence of the risk and act depending on your tolerance for risk. If the risk is too much for you, don't start the business.
As Krzysztof said, you can't stop them form copying and there is the risk of them cutting you out of the chain altogether.
Deciding if this risk is worth it for you is something only you can decide. Although there are some things you can do to minimise the risk to yourself.
Appeal to the customers Appealing to the customers means you are more likely to make sales, more likely to make sales means more likely to be profitable, being profitable means distributors are more likely to take the risk of selling your product, maybe even alongside there own, this is done frequently!
So how do I be more appealing to customers? Fair Trade Well look around at the current situation with food, some people will go out of their way to buy fair trade food, even though it is more expensive, because it appeals to their ideals, they want to support the farmers, if you can do a similar thing with your product you will appeal to a large group of customers.
GM Foods Like it or not if some of your crops / ingredients are genetically modified you will instantly lose some of the market, some people just don't like the idea of genetically modified foods and will go out of their way to avoid it, if possible for you I suggest trying to do the same, at least until you have a large customer base to take that risk.
Cost If your product is going to be sold year round you might want to try to decrease costs of production to allow you to sell at a lower price and encourage more buyers.
If it is something like strawberries, a 'luxury' item because of its limited availability then you can probably charge a little bit more and still retain the same customer base.
Health If your food is healthy, and not genetically modified then you can also sell in health stores, giving you more chances for success and more distribution points.
Supposedly people who travel to health stores are also more likely to travel up to 15 times as far to get their health foods.
"People will only travel a mile or so to visit a convenience store, but health food stores can see customers travelling up to 15 miles"Get well known If the population at large knows who you are and knows your products then you can create buzz around your product, this means that big stores are more likely to stock your product because they want a cut of whatever profits you are about to get from the current media hype
Kim Power, Lakey and Co (business brokers)
Conclusion Following these points and making yourself as profitable as possibble means that the big stores are more likely to stock your product, even if they will copy it and sell it themselves, this is because people like choice, and stores will want a cut of the profit of the people who buy your product.
For further information see this question and my answer here: Why do retail outlets not stop stocking a product they've copied?