The copyshop model might work out really well. Perhaps cross-pollinated with the MacDonald's model, ideally not by making the entire site run under e.g. Shapeways flag but less granular: "Mike's materializers" around the corner might have brand A processes x, y and z available, and brand B processes w and x, whereas "Jen's stuffmakers" further north has u through y from B but only y and z from A. The model directories list compatible sites near you with each model.