Once you get one you can just let it grow and then cross breed from it. A
square crops designs is good for this. With my experience the most
likely plants to get from a cross breed is one of the parent plants
followed by a similar plant and finally a completely different
plant(flowers being the most common of these).
So breeding reeds together is the easiest way to get stickreed and wheat for redwheat.
In other news: Pretty much only reed will grow in the desert.