Another source might be a Latin-American grocery, if you have one locally. While most Mexican tamales are wrapped in corn husks, banana leaves are more commonly used in the Yucatan and Guatemala (and maybe other places too.) Our Guatemalan friend makes us a batch every few months (delicious) and I know she just buys the banana leaves at her local shop here.

Just found this source: http://grocerythai.com/banana-leaves-thong-p-198.html