You can by pre-made leading edge stock that is milled to a rounded profile. Sometimes you get lucky and it is the exact profile you need or sometimes you may have to tweak it. In other cases you use square or rectangular stock for the leading edge then carve and sand it to the profile you need. It isn't as hard as it sounds; balsa is a very workable wood. I've made a gauge from plywood with a cut-out that matches the leading edge profile then used this to check my work as I do the shaping.

Those are just two methods. there are a lot of builders out there and many techniques.

You might want to build a kit or two before jumping into building from plans. That way you can get a feel for some of the construction methods, materials and so on.<br type="_moz" />