Hi David
The sprockets will likely have to come from a Lav supplier (OCT, Wagner etc) but you can save on post and buy a good quality chain (I use o-ring) from any local bike shop.
Be careful fitting the front sprocket, some are a tight fit and bashing it onto the countershaft splines can push the stub-shaft into the gearbox a bit and give you a motor strip job. I heated mine up and carefully tapped around and around with a light hammer, it barely started on the splines from cold.