I got my OEM thermostat from Partshop here for only $21, I think my 400E uses the same thermostat as your W140's.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.