Your best bet for this part will be a swap meet. If you mesure the seat post and handle bars I can take a look and see if I have one that will fit, it's yours for the cost of shipping.
The stoker stem on your Columbia is a heavy steel part, I don't know of anyone that makes one that will fit. You may want to ask Mark, if someone makes one that will fit your seat post he will know.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." - Arthur Conan Doyle