The problem is not obtaining the APK, that's the easy part. The hard part with a lot of online banking apps (as well as apps like Netflix) is that they use (and require) Google SafetyNet to ensure that your ROM is "safe" (i.e. hasn't been tampered with). The guys over at the MicroG project are trying to find ways around that but it's really a cat-and-mouse game. Last time I tried (which, admittedly, was quite a while ago), neither my online banking app nor Netflix worked for me on a custom ROM.