I _think_ the bulb holders for a 111 coupe are the same as for a Pagoda. So expensive. There is a really nice complete assembly on ebay right now for what I think is a bargain:
Mercedes W111 W112 Coupe Cabriolet Convertible Tail Light Taillight 280SE 3 5 | eBay

I can't imagine you getting a good bulb older for less than that. But it might not be the correct side...