Best source to obtain a MS app (Edge) APK file

I won’t trust any except the ones directly provided by the manufacturers.
The only other thing you could try is to get the apk extracted yourself from a phone where it’s installed already (there are some apps which can do that). But you can run into issues as sometimes apps use more than one single apk or have model/version specific installations which can’t be provided this way.

I would maybe more likely recommend to investigate if a different browser (which provides apk) might work or if you could workaround this restricition by proxy (i have no idea how strict china is about all of that) or just use chrome which should be there by default.

What ​@Moombas says! There are enough places where you can get an APK file, APK mirror is a good one. But since you will never be able to verify its source you will never have the guarantee that the APK file is safe and malware free. I’ve used it personally without issues, but would never ever use it in a business environment. 

While it’s still a bit scary to trust, Microsoft does list a few App repositories in China that host the Edge APK file. 

https://learn.microsoft.com/en-us/deployedge/microsoft-edge-install-mobile-china

Still use at your own risk I would say.