Default [Q] Catch APK before install

Hi there, I would like to know if it is possible to catch/save APK's before they install. (Not relating to Play Store)

For example, when I download an app off the Amazon Appstore, it says "downloading" before opening a prompt for me to scan the file before pening the apk. Does anyone know where the apps apk's are downloaded too when downloaded from these alternative markets, or where get stored before they get opened?

Maybe their is an app that appears in the prompt before the install, which allows me to save the apk?

Many Thanks