What are my rights with against a company that my small business gets merchantdise from, after being told that my account is being closed

As a general matter, it is lawful for sellers to enter into exclusive-dealing contracts with distributors of their products, to the exclusion of others. Generally speaking, the antitrust laws of the United States preserve the right of sellers to choose the parties with whom they will do business. Your inquiry does not set forth any facts that would constitute an antitrust violation.