You will find all your answers and more with a little search and digging.
Notwithstanding, you or a surrogate may get an APK directly from Google Play with an Android device or emulator, from other repositories like 1mobile for free ones, fdroid for FOSS, or directly from dev. Other sources may be illegal and have malware.
You may convert APKs to BARs by obtaining the necessary apps and keys from RIM to do it yourself, ask Koz or others here to do it for you, or upload to Unkers website for conversion.
To get converted BARs, visit Koz's site, or App World for pre-tested ones, or search the forum.
Lastly, android apps may use code libraries that the PB does not support, so some apps will not work, while some work partially. The PB is also finicky and may require 3 - 4 reboots before the app works, or that older apk installs before a major PB OS update, may require you to do the annoying wipe.
You will only be able to upgrade in android app if it came directly from App World. Android apps as you would expect, expects to use an Android app manager to download and install updates- the PB has none unless rooted.