It's a simple question, but I can't find an answer anywhere..
Can I use AdMob ads in my apps on Amazon Appstore? If not, what ad networks can I use?
AdMob is an ad network which supports (in this context) Android applications. the Amazon Appstore is merely another platform for developers to release their applications on.
So to answer you question briefly, YES, you can use the AdMob Api, integrate into your Android app and release it on the Amazon Appstore, or Appstore for that matter (naturally, the ones that support Android apps. ;-) )