I went to the local automotive/hardware store (Canadian Tire) and got a roll of black humidifier filter foam. It was the right width, just had to cut it to proper length, used spray adhesive on the foam, then stuck it in place until the material held. It was a bit thicker (1/4") than the original foam, but still worked very well when the access panel was screwed back in place.
The part number for the Canadian Tire humidifier evaporator pad was 43-5237-6.
The price was $6.29 + taxes. This was a few years ago (2011), so not sure if it's still available.
[This message has been edited by canfirst (edited 05-20-2014).]