Antitrust lawsuits can be based on either federal or state law. If the former, sue in federal court in Kansas (U.S. District Court for the District of Kansas). If under state law, you sue in the Kansas courts of general jurisdiction (the Kansas district courts). Pleading and proving and unlawful antitrust conspiracy, which is the legal term for horizontal collusion, is tricky, so your job in representing yourself pro se will be very, very difficult.