-http://www.4woman.gov/Breastfeeding/index.cfm?page=227
-This one
kellymom.com :: Extended Breastfeeding Fact Sheet
it specifically says at the beginning:
"Although there has been little research done on children who breastfeed beyond the age of two, the available information indicates that breastfeeding continues to be a valuable source of nutrition and disease protection for as long as breastfeeding continues."
-101 Reasons to Breastfeed
ProMoM, Inc. - 101 Reasons to Breastfeed Your Child
-Here's a good one
Why breastfeed? The health, emotional and environmental benefits of breastfeeding, the powers of breastmilk the hazards of formula, and more. Breastmilk is the perfect food!
-Another good article
Breastfeeding benefits mother, baby - everyone! | coshoctontribune.com | Coshocton Tribune
First question on this website for supplementing formula while breastfeeding:
Supplementing with formula | BabyCenter Reasons Why Breastfeeding Women Supplement with Formula
It's hard to find things on supplementing formula and breastfeeding, hopefully someone else can help better on that one.