Trek 8000 seat tube QR?
 

I recently bought a circa 2003 Trek 8000 mtb that is missing some parts. I have managed to put it all back together, however I am missing the quick release seat post hardware. I need the actual clamp that goes around the seat tube.

Where might I find something that will fit, or a OEM trek unit?

You can measure the outside diameter of the seat tube with a caliper and order a clamp from many online vendors, either quick-release or bolted.

see your local trek dealer.