Has anyone installed shoulder belts on a 1990 Grand Caravan?

I'm in Canada and the dealership say they won't do it. But my kids have outgrown their carseats and I'm not comfortable having them in lap belts only! I'm in Canada and the 5 point carseats here only go to 48 lbs. Has anyone been able to do this themselves? Or know of how to go about finding someone that might be able to do this aftermarket? Is it even possible with this model year? Buying a new van right now is out of the questions - plus we had our whole engine rebuilt 18 months ago and replaced the transmission in the van 2 years ago - so we have a lot into this van already. (Can't believe I didn't think about the seatbelt issue).Thanks for any help anyone can give.

Cheryl