I'm not sure you can buy the radiator tanks separately, but, even if you could, the repair may not save you much money over the cost of a new rad.
The only lasting solution is the Zionsville rad, an expensive alternate. Some on these forums have opted for it.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.