I found this on ifisher.com - may be worth a try:

We went car shopping, and were interested in a used 2000 BMW X5. The belt wasn't long enough, so we spoke to the BMW service/parts department. They found out that the X5 was available with an "extended length" seat belt, but BMW of North America said that ALL X5s in the US already came with this extended length seatbelt as standard equipment. (I'd hate to see the short one then!) We traced the outline of the buckle, and went looking at seat belts on all the American car dealers (Ford, Chevy, Buick, etc.) We found that the some versions of the Chevy Tahoe seemed to have a compatible buckle, and the Chevy dealer gave us a couple of extenders to try on the BMW. One of them was a tight fit, and seemed to work. It fit better after we filed away a bit of the plastic trim, being careful not to touch any of the metal which provides the strength of the buckle. (We returned the other extenders which didn't fit to the Chevy dealer.) The Chevy dealer said that there were at least two or three different part numbers, so anyone ordering might want to order all of the possibilities and plan on returning the ones that don't work, rather than ordering one at a time and waiting. ---Jonathan