I need a lawyer to write me a letter

I am a freelancer and an agency I used to work with hasn’t paid me what i am owed and I have been going back and forth with them for a year now. It has been such a stressful and draining time and I just want a letter to be sent to the agency so they know that if they don’t pay what is owed this will go furthur. I know they have my money they just need to transfer it.