Only by going through the web browser, which apple can't now shut down without alienating too many users. If they thought they could get away with it, they would.

They have actively worked to disable any other store - for example, the kindle app cannot allow purchases or even link to the kindle store online, you have to manually search out links online instead, then download. It's absurd, not in the users' interests, and an abuse of Apple's power over the platform, but that's what Apple require.