Get a hold of ZeroDog on the forums here.
He has supplied quite a few front and rear springs for 640E and 640Adv owners.
I have ordered some springs from him a couple of weeks ago and just waiting for them to arrive in NZ. The rears get made to order. About a 2 weeks delay.
ZeroDog sorted the springs for another forum member called bmwktmbill before he rode though europe/asia.