Good question. I've never bought any myself. I save left over parts from kits and often, for me, it is the clear parts. I only recently ran out on the M4A1 build. But I have six pair of Tamiya head lights on allied tanks but I only have 3 Tammy US rigs. My guess is my brother gifted me some at some point. Painlesswolf has bought "trees" from a company out east (but the name escapes me).
But as you say, the Panther is finished. And it is a stunning build.