Ken,
I'm in the same boat and have been looking for several months. MB will sell you one for just under $1K. I haven't found any aftermarket suppliers. Millers lists them, but when I called to inquire, the said they hadn't had one in some time and could get one from the German supplier for $900+ (i guess their German supplier is MB).
Be careful of used ones. I found a used one that had supposedly been pressure tested, only to find out after I installed it that it leaked as much as my original failing one. I took my old one and the used one I had found to a local specialty welding/radiator shop and was told both cores were leaking (most likely due to corrosion) and could not be repaired.
Please, please, please, let me know if you find a source, and I will do the same.
sven