Should be a pretty simple model to build without detailed plans. I did a Google search for "narrrowboat plans" & found many layout plans in addition to scores of photos including several models.
That said, Waverly offers a hull & plans:
http://www.waverleymodels.co.uk/canalbarge.htm
Riverside Models offers several kits if you don't want to do a scratch build including a new 1:12 kit featured in the May 2012 issue of Modelboats.
http://www.riversidemodels.co.uk