I bought a rad core for my D2 5U from Rocore in Franklin, Wisconsin. They had the D2 core in stock. Their contact no is 414 421 4666. The person I spoke with was a lady called Cindy Frost who was extremely helpful.
This company's details were given to me by a kind person responding to my request for information re D2 radiator core on the ACMOC site, hope it helps you out!!
"I became a member recently because the wealth of knowledge here is priceless." - Chris R
"I also joined a year ago. had been on here a couple of times as a non-member and found the info very helpful so I got a one year subscription (not very expensive at all) to try it out. I really like all the resources on here so I just got a three year. I think its a very small price for what you can get out of this site." - Jason N