Which watch do you have now? I see that it's not available on either of my watches (Fossil Gen 6, Citizen CZ Smart Gen 2), both of which are running Wear OS 3.0. It's possible Pandora hasn't bothered to update its app to run on Wear OS 3.0 yet.
Even if you did find the older apk, there's a good chance it wouldn't work well -- it might constantly prompt you to update, or inform you that it is no longer supported.