My 8 year old Grandson's Mother has been out of his life almost since birth and the Grandfather and I have raised him. Our son lives with us and has custody, the mom is a drug abuser and in and out of jail.
Our son can not care for him if he moves out of our house, which we want.
With consent from both parents, how would I go about getting guardianship.

Family court... that way you can get legal custody.

Or consult a Family Law attorney