Canvass Material - for tents or tarps (Ashurst Wood RH19)

I am really in need some canvass to make repairs to old tents and tarps. This is mainly for a charity Sea cadet unit but also for some of my personal projects. So, please help if you have any old canvass tents or tarps or just some spare canvass material laying around, it will really help. Ideally it needs to be heavy duty and waterproof/resistant but I can waterproof myself though.