I think you should look for an attorney who offers a free consultation, and ask if you have a lawsuit for slander. The attorney can advise you about whether whatever these people are saying about you is actually slander, about whether you can prove damages, etc.
Whether you have a lawsuit to pursue or not, that attorney may be able to take some action on your behalf; and s/he may be able to advise you about any government, licensing, or other agencies available for addressing disputes/complaints in the context of "dealings" between businesses.
It's not clear what your situation is, but you could also try contacting (in writing) someone at the company who deals with people who work in your capacity. Slander means someone is lying, so you could try addressing the lies, pointing out why they are lies, and trying to address your problem that way. If one person isn't helpful ask to talk to someone else. Keep records of all letters or calls, names, etc., and write down who says what. You'll then have a record of anyone's refusal to try to clear up any "misunderstandings" to give to an attorney if necessary.