My 2 year old son's mother has a drug problem. Several times in the past she has taken my son and disappeared for months at a time. Last summer she apologized and told me that she wasn't on drugs anymore. She asked me to give her another chance at a relationship. I took her back because I loved her and wanted our son to have a mother and father. For about 6 or 7 months she worked and helped with our son. Suddenly in January she called me and told me that she was quitting her job. She quit her job and progressed back to using drugs. I worked and was with son all the time because she started leaving every day and returning around 12am. I confronted her about every thing and we had an argument. She told me that I had to move out and I did. I still take care of my son 100% financially and I pay her rent. About two weeks later we got back together. One night I came to visit her and she let another man into the house and wouldn't let me come in even though I pay the rent. I decided to focus completely on taking care of my son. Last Thursday I dropped my son off and discovered that she was having a drug party in front of my child. I called the police because I didn't know what to do. The police came over and found evidence of her doing drugs. The officer called me and gave me a case number and informed me to contact DEFACS the next morning. Out of spite she refuses to let me or my family see my son. She taunts us if she sees us and brags that she will make me pay child support and that I will never see my son. What can I do to get custody of my son in this case?

No, CS and custody/visitation do not depend upon each other. As a matter of fact, assuming there is no custody order in place, you have equal custody rights with the child's mother.
You need to get an attorney and seek custody throug a custody order.

I'm a little unclear here. The police found evidence of drug use and allowed the child to remain in that house?

Did you contact DEFACS the next morning? Did you ask that the child be removed from the mother and placed with you until an investigation is completed? What did DEFACS do?

Assuming that they are not doing what they should be doing to protect the child. You should be filing for full custody. You should be looking for an attorney and filing for emergency custody Monday morning or at least once you get an attorney. File yourself if you can't find (or afford) an attorney at this point.