Lots of set options and good prices here; http://www.vintagecomp.com/ . This site is located in LA, but aimed at the Japanese market so it is in Japanese. Shoot the admin a mail at vcsales@vintagecomp.com, he will reply quickly. Just for reference he has a 24GB 3 chip (3 x 8GB) set of ECC registered for about $242
(http://www.vintagecomp.com/shop/shopdisplayproducts.asp?id=738)
Usual disclaimer: I have absolutely no connection with this company, just a satisfied customer.
Your empty slot is likely #4 or 8 and in that case I thought there had been a guideline to match it to the shared #3 (or 7) slot - though that may have changed or be a minor issue today.