http://www.kellymom.com/menu-herbal.html
This is a link that i use a lot to check on safety of supplements while breastfeeding.

It doesnt make any sense to me at all that something like green tea (that pregnant and nursing mothers have been drinking for millenia) wouldnt be safe to use while breastfeeding. I think that if you check any supplement it will usually carry the blanket warning statement "Not for use during pregnancy and lactation unless directed by a healthcare practitioner". I think it's more of a liability thing than an indication that the supplement has been researched and found unsafe.

Curious what you want to take the green tea supplement for though...maybe there's something else you can use instead if you dont feel comfortable taking green tea.