I am looking for plastic C-channel edging to finish an access hole in 3/8 inch plywood. Does anyone have any sources for this? Canadian sources would be nice, but internet sources welcome. Please and thank you.

Here is some that will fit your sizes.

JC Whitey - Edge Trim

Cam,
You might check a local auto glass repair shop. I've found a variety of widths and they probably will sell short lengths if that's what you need.
Kurt & Ann K.

Also try the Kenworth, Peterbuilt, Mack & White truck Parts Departments. I found lots of useful stuff in them.

When I was rebuilding my Love Bug, I found that McMaster-Carr was a great source for edge trim. Their on-line catalog is great once you figure out how to use it. Trim-Lock is the brand and they have just about every version made. I could not find anything about the belly band on my Love Bug so I ended up using some of the larger Trim Lock to do the job. It looks great. I also used a difference variety to finish the lower edge of the trailer.
Just measure your thicknesses and the wideth that you want and look at their examples.

Trim-Lok and other moldings from the Helpful-Links page:
Moldings

Home Depot and Lowes sell a white c channel molding (2 sizes) in hard plastic to put on the edge of drywall where it might but up against brick or something else. It is a finish product. Very cheap.

It is straight and it does not bend and I have used it for several things.