Look on Ebay here:
eBay Motors: Mercedes ML 3rd Row Seatbelts & Brackets Set 163 430ML (item 260098472901 end time Oct-14-07 06:38:37 PDT)
not sure if these are OEM but he sells a lot of other MB stuff and I contacted some of his buyers directly for feedback and they were all happy with the product.
This is definitely cheaper and easier then getting them fabricated I would say.