Here are some things I'd try.
1. There are locations scattered around the US that specialize in collecting and parting out BMWs. Several in the Rancho Cordova area near Sacramento, and Lone Star Auto Parts near Houston to name 2. I've always felt they under advertise themselves, so you could call them.
2. Ask your local upholster what he has that is close and if he can stitch it in. Then call FibreNew and have them dye it. They might scuff up all the red leather in the seat so it's more uniform and just match it to the backrest red leather. I used them to dye some worn areas on a seat (not as big of damage as yours) and the results were excellent.
www.fibrenew.com
3. Find a full seat bottom in Oregon leather in any color and ask if the middle color can be re-dyed, as the bolsters will still be black. I'm pretty sure it can.
4. And as you've probably done, keep an eye on Craigslist, Ebay, etc. Even if you find a replacement, the seller won't sell you just the bottom; you'll have to buy the whole seat.