Any standard 16x25 filter will work but they may not sit in the track properly. You can order a new fiber filter online, look for "hogs hair filters"

 

Exactly as you said -- thanks. The usual "framed" filter didn't fit the rails and it actually has to be flexible anyway because of the way my genius contractor ran a pipe right in front of that area. Thankfully, our McGuckin carries pretty cheap cuttable fiber sheets.
By the way, how often do these have to be replaced (not washed) before they start shedding crap all over? Again, thanks.

 

Should be checked every month and washed as needed. How long it lasts will depend on how often it gets cleaned.