Gainging custody of my son when the mother is a drug addict

Hey im new here and have never done this before... with that said... My husband and I have his 4 year old son in our care and he lives with us. Him and his ex gf have joint custody but its only a noterized piece of paper and nothing thru the courts. The mother is a drug addict and we have been keeping her away from him since she isnt fit to take care of him and only wants him when she needs money or wants to be close to my husband... not going to happen.. she lives with her father who is a registered sex offender and smokes in the house. Her bf stays there too who once hit my step son and she allows him to be around her son. which is why we keep him away too. Her and her fathers house is not safe for our boy and we want to keep him safe. I have called the police and they say we dont have to let her see him since its a civil matter and nothing is in the courts. We know she is on drugs.. she shows up at my husbands work and refuses to leave till she has money, and he says no and just ignores her and then she finally leaves.. just the other day she showed up with needle marks all over her arms and my husband wouldnt give her money so she called the guy who she cheated on my hunny with who she said could also be the father and tried to tell him he was the father and needed money from him... she was using her own son to get money for drugs!! we were so mad when we found that out... she is also claiming him on her foodstamps andhas never bought him food once. yes i have reported her.. her mother has a forwarded message on her phone from a family member asking her for pills in exchange for food stamps... its a voicemail that says " hi aunt *** this is ****** and i want this many pills in exchange for this much in foodstamps"... so we do have proof and some evidence. i know this is a long and confusing story but we need to go to court and get custody of him so she cant have him and use him for her addiction. its not fair to her son and we want to keep him safe. our boy has lived with his grandparents (the mothers mom and step dad) for most his life but now my hunny and i have him... he is happy and healthy and learning so much. Yes the mother has a lot of problems and most people want to know "whats dads story".... no he wasnt always around for his son but that wasnt his fault, the mother never allowed him to see his son and he worked so hard just to pay child support which never went to his son, the mom spent it on drugs then called him to say they were out of food and diapers already.... My husband is a great man, no drugs, no drinking, no smoking, nothing.. he just works his butt off to support me and my unborn baby and his son. He works at a car dealership and we are also bail enforcement agents (bounty hunters and bail bond agents) we respect the law and know a lot of people in law enforcement. We are truly the best home for this boy and love him too much to let his mother get a hold of him and use and abuse him. we need to know how to go about this in court, what to do and who to hire and what we need to do it. we feel we will win this but dont wanna be cocky or too confident. we have plenty of people to help us but we cant afford a pricey lawyer... can we represent ourselves or will that be hard? please no stupid responses or anything like that. im a concerned parent who wants whats best for my childs safety and well being. thanks