Try a Wide Neck Carboy Cap on eBay.
http://www.ebay.com/itm/WIDE-NECK-CARBOY-CAP-138mm-Wide-Rubber-Bung-Large-Bung-Demijohn-Rubber-Bung-/222236572989?hash=item33be54fd3d:g:LvYAAOSwGtRXyBN-
I just ordered one to fit my Demijohn. It is $16 tho....about half of what I paid for the demijohn.