You looking for a stock replacement, or other? Try Autohausaz.com, getbmwparts.com, penskeparts.com, bimmerworld.com, turnermotorsport.com, pelicanparts.com, bavauto.com, and anybody advertising in the Roundel. If you want a solution to the shortfalls of the oem products, check http://www.zionsvilleautosport.com/store/screen/prod/store_code/6134/product_code/E39SDCKII.htm http://www.zionsvilleautosport.com/store/screen/prod/store_code/6134/product_code/NP-ALLOYRAD39.htm
I know this is a bit of an old topic, however, if you are still researching this, another option might be worth a look zionsvilleautosport.com. An all aluminum radiator from Zionsville Autosport. I know that some people may balk at $700 for a radiator, but I plan on keeping my E39 till the wheels fall off. Whether it is an OEM, Behr or Nissens radiator, they all have plastic end tanks. A $150 radiator is nice, but your car is still down for a while when your plastic cracks.