Filter Replacement/Questions

I am trying to replace the filter for my furnace/AC and I can't find one anywhere. I have a Coleman furnace/ac package unit. It is a Flanders Corp filter, 24x42. I have been in contact with their customer service via email and phone; they were less than helpful. The previous owner had a note on the filter saying wash once a month. I don't believe this filter is washable?

I've been looking at a couple web sites for custom made filters but still having a hard time finding a one. I attached a couple photos. Any advice would be much appreciated!! Thanks