Which fretless neck?
 

RBX260f
http://usa.yamaha.com/products/music...0f/?mode=model
According to the specs on the Yamaha website, the radius is 10"/250mm.
What else would i need to measure to find a new neck? Or does anyone know off hand what would fit? Any particularly good websites?

The radius of the fingerboard has no bearing on whether a neck will fit; the scale length, and the size/shape of the neck pocket, do.