Best place to purchase a new drive shaft?

The universal joints went out in my 1991 735IL, and as all of you have found out, "no replacing them"! Does anyone know of a cheaper place to purchase one? The cheapest I can find is $450, and I'm not really sure that it's the right one. How do you know which one is correct? This is the first major thing that has gone out in my car that I haven't been able to fix by myself with major forum help!!!

Thanks,
Mike

Best place to purchase a new drive shaft?