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.
__________________
Bentbaggerlen
"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