There is a lot of bad advice in this thread.

For starters, exclusive distribution deals are by no means illegal. In fact they are extremely common. Sometimes there are very good reasons for them, sometimes not, but it matters very little what ought to be.

Second, your idea of waging a PR war against the other store is probably a bad idea. The "back end" of business is all relationships and reputation. If you're not familiar with it, it sounds weird, but the people who work for and even own direct competitors tend to have very congenial, even cooperative relationships. They belong to the same trade groups, they go to the same shows and seminars, they frequently switch jobs to work for their former rivals, etc. Be careful about burning bridges or making waves that could ripple back to your vendors and suppliers: they have tremendous power to help or hurt you based on relationships and reputation, as you have seen.

Third, unless I misread your re-cap, you should not be doing anything about this, except at the instruction of (or at least with the consent of) the owner.

With that out of the way, the first and maybe the only thing to do is to sit down with each of your supplier(s), with the highest-level person you can get (and yes, sit down with them, do not try to negotiate via phone or email), and explain the situation to them, something like this:

"We appreciate the help you guys have given us so far, but we really need a couple of things that we're not getting from any of our suppliers right now. One is certainty about our product lines. We can't run a business with half-full racks of mismatched jeans in odd sizes, which is what we've got now that we've been cut off of (brand X). So we need some assurance that we're building permanent relationships and supply chains, not just buying dribs and drabs of whatever fell off a truck this week. The second is access to a full selection of brands and product lines: we can't build any leverage or economy of scale buying 6 units from you and 3 from the guy up the road and 5 from the other guy. We need to consolidate, as much as possible, around a small number of reliable suppliers who want a mutual partnership."

Then you see where it goes. But do it with all your cards on the table, and do it businesslike and cordial. See what they come up with, what they can offer. Don't try to play a game of wits, and definitely don't threaten or throw tantrums that it's not fair and you're going to tell on them if they don't share with you...

Just explain what you need and let them know that you're shopping for a supplier who can give that to you. And let them know that you are not just willing, but actively seeking to cut down the number of suppliers you do business with to increase your leverage and relationship with the ones who are ready to partner with you long-term. And let them know that price is important, but it's not the only factor: you'd rather have one reliable supplier who always returns your calls than haggle over nickels every other week when the other guy offers you a lowball deal to get you to open an account with them.

And don't just say all that-- fucking mean it, for their sake and for yours. If you run all your suppliers and potential suppliers through that test, you'll probably find that one or two are willing to roll the dice and go above and beyond for you. But if they find out you're buying from a competitor when it's a penny cheaper or whatever, then you're back to square one. People value those relationships, and whatever their competitor can offer a lower price on this week is probably the only item they're making a profit on this week.

Your problem is not a fashion problem, and not a fairness problem nor an issue of social justice or civil rights: it's a business problem. Handle it like a businessman.