Are you wanting to weigh your baby in addition to well checks or in lieu of them? Because if you are trying to avoid the well visits altogether, I would encourage you to rethink this plan. A doctor evaluates much more than weight.
Now if you are just wanting to keep a close eye in between medical visits, I am not certain where you could get an accurate weight. Produce scales are notoriously not balanced. Cheryl's trick of holding the baby and subtracting will work for general weights, but I don't think you'll get the specifics with ounces. I know when my son was having weight issues, the office helped me track his weight with weight check appointments. I was not charged for these visits. The nurses simply weighed him and charted it for the doctor.