If baby is consistently gaining weight, there shouldn’t be any reason to worry and certainly no reason to supplement. Growth charts aren't a contest — a baby in the 75th percentile isn't necessarily any healthier than a baby in the 25th percentile.
Let's say baby is in the 15th percentile on the weight charts; that means about 15 percent of other babies weigh the same as or less than her. Your doctor will probably expect her to stay near that 15th percentile “curve” as she grows. (Make sure your doctor has access to a growth chart for breastfed babies — they grow differently from formula-fed kids.) A low percentile is just fine, as long as baby continues to gain weight, grow in length and head circumference, and is otherwise healthy, happy, and meeting her developmental milestones.
If you aren't just paranoid about percentages, but have other reasons for worrying that baby isn't getting enough milk, talk to your doctor and get hands-on breastfeeding help from an experienced, certified lactation consultant.