Rudders can be a challenge but they do not have to be that difficult to make. A scale rudder for a model warship is almost impossible to buy from a hobby shop so I have found that making them is a necessity in most cases.
I make all my rudders from acrylic sheet (Perspex) of an appropriate thickness cut to the shape that I am after and then sand either side to form and aerofoil shape. I drill a hole for the rudder post and glue in a metal post with superglue. Sometimes it is necessary to glue two pieces of perspex together to get a piece of the right thickness. This often helps to keep things even when sanding to shape as the glue line between the two pieces acts as a visual center line.