What is the age, brand, and model of your furnace?...you may have to get a custom made replacement.

TD

Looking at the Canadian place I buy from, it appears that size would probably be generically listed as "16x25x3".

An example of close would be Lennox XO591 or equivalent ( 15 7/8" x 24 1/4" x 3" ).

That is a strange size. Are the actual dimensions the same as what you show in the photo?

Is is there a filter box on the side of the furnace? If so it there a brand and model number? That may give a clue as to what filter size filter is specified.

That's the exact size - it fits a 16x25x4 "nominal size" filter box. It's an industry standard size:

https://www.amazon.com/Filtrete-Allergen-Attracts-Particles-Guaranteed/dp/B00OPSIIGQ/ref=sr_1_7?dchild=1&gclid=CjwKCAjwrcH3BRApEiwAxjdPTe_eiViag8V_NIfXUX0CLcxLxbAWvaPR0mJsL2iAdo_Bd1ow7yCItRoCRW8QAvD_BwE&hvadid=176949532760&hvdev=c&hvlocphy=9009671&hvnetw=g&hvqmt=e&hvrand=18333237139867742836&hvtargid=kwd-27427599315&hydadcr=13859_9624423&keywords=honeywell+cf100a1009&qid=1592877317&sr=8-7&tag=googhydr-20

These will likely work: https://www.amazon.ca/GeneralAire-ReservePro-16x25x3-furnace-filter/dp/B015225GEA. It might help to compare some of the other details on the box if you want to be sure a new one will fit.

Twintech Heating & Cooling

https://twintechheating.ca/

"That is a strange size."

Nope.

Standard size 16"X25"X3"