until now, we only allowed installation of apps from the Managed Google Play Store. This worked fine, but now there is an app that we need that is not available on the Play Store, but only from the provider's website.
We don't want to generally allow installation of apps from unknown sources.
I can't find any way to do this, but I also can't imagine that this is such an uncommon occurance. The provider does not want to put their app on the Play Store as it can only be used by customers who have a contract with them.
All of the devices we use run Android.