The best formula for breastfed babies? How?

arcook0307

Our almost 10 month old is not gaining weight rapidly enough.  I suspect it may be my breast milk.  She doesn't nurse for very long and only 5-6x a day.  Granted she's eating 3-4 solid meals a day.  I just have the suspicion that she isn't getting enough milk.  It was recommended that we go on a formula supplement but I was given no guidance. I remember something being in the news about formula but I need to go get some ASAP to help put some weight on her.  Even though she eats constantly she is barely on the chart for weight at all for her age.  Favorite formulas??  And how to go about introducing it?