I have a Ducane Furnace Model # fpbb050a3. One website informed me the size of the filter is 23x13x1/2.
As best as I can measure, that is accurate. I cannot find this filter anywhere.
Does anyone know where I can find one of these OR how to make one?

There are adjustible permanent electrostatic filters made that you can cut and snap the frames to fit any size smaller than they are from the store. Most home improvement stores should carry them. You just need to flush them under running water a few times a year and reinstall them. IF you happen to be in the USA.. Home Depot or Lowe's would carry them as only two examples.

In my case the idiots that installed my furnace (before I bought the house) never left a provision for a filter... the only way I could rig up something left me with an odd size filter and it worked in my case.

Did you go to the Ducane website? Or find a Ducane dealer for the filter.