Rails for these old cases are often difficult or impossible to find from anywhere. And many old used cases miss all or some of their rails.

You might want to search ebay and equivalent local sales sites if there happens to be something available, but the most likely route you have to take is to probably to 3D print them. First, search thingiverse amd printables if someone already had designed rails for that particular case. If not, it is time to take some measurements of the originals and design equivalent (not necessarily identical, just ones that do the job) rails with CAD software of your choice and print those.

Final option is of course just trying to come up with some sort of hack that does the job. Here we are talking about things such as wood blocks, zip ties etc, rails from other cases etc.