Read the FAQ on rebuilding the heater box.
http://www.bmw2002faq.com/content/view/73/32/
Find a good doner heater box and rebuild it using good proper parts (new cables, pressure checked or rebuilt core, and a good non-leaking valve, new BMW hoses, rubber seals and clamps)
Not sure if they have more, but I bought a couple of the little bolts (Heater Valve Cable Bolt at Valve) from Jaymic in the U.K. recently.
http://www.jaymic.com/shop/parts_02_products.asp?grp=02CO&page=5
Take your time on the rebuild. Do it right you'll be glad you did. Then install in your car on a weekend when you have time.