Rob Gillum at Coherent Source Service might know. And some ebay, etc. resellers sometimes salvage old gear. Also, Madisound was still supplying some parts in the mid 80s and may have a clue. It was probably made (I don't remember for certain) by Vifa. But a stock driver will not work. The motor was custom designed and everything else was specified by Thiel. Normal woofers don't have the smooth high-end rollout needed for first order slopes.