If the window was from the 90s, they probably have retired the model by now anyway. No one sells the same model of anything for 20+ years. Even if they did have replacement sashes for it at one point in time, they may have decided a while back that they can no longer justify the cost of manufacturing replacement parts for older models from 20+ years ago, especially when they've got these brand spanking new windows with all these spiffy new features that they want people to buy.
Sorry, man, but unless you know how to make a vinyl window sash to replace the one that's busted then you're stuck. You could always start a company that manufactures third party replacement parts for old vinyl windows. It seems there might be a market for it.
And they don't have to do anything if they don't wanna!