You'll need to buy a used iPhone to use that app, since it isn't available for Android (I've tried to find it too, after seeing Steve O'Nions using it on his YouTube channel). Since you don't need cell service on this phone you can get an older one (a 6 or 7 or later should be able to run a reasonably current iOS), which can be had on eBay fairly inexpensively (GameStop used to sell used iPhones, too, though I don't know if they still do). The best part of this is that since you won't be using the phone as a phone it doesn't matter whether it's a Verizon model or non-Verizon (they're incompatible at the voice call level, but otherwise identical). You can use wifi connectivity to access the Apple Store to get the app, as well as for other things you might want to do with a spare phone.
There are apps that require a connection to run, but almost all of those are ad-supported, so if you've paid for the app, it should work when disconnected (and Steve O. uses his in place where I'm pretty sure he has no signal).