A couple of days ago I stumbled across an article talking about this app Pay Finders, which dedicates itself to indexing and mapping as many Apple-Pay-Accepted locations as possible. I tried it out, and it's pretty cool.
The app is crowd sourced, which means it relies on the users to submit new places that accept Apple Pay, or report old ones that don't anymore. There already seems to be a pretty big database a couple of months in - I'd call that a good sign.
The app works pretty well, and aside from pretty janky UI styling and a lack of optimization for the iPhone(s) 6, the app shows a lot of promise.
Check it out if you're really into Apple Pay, it ought to make your life a whole lot easier.