I don’t like to install beta versions of iOS on my daily driver device so I tend to use old devices I have left around from years past. This year, however, I didn’t have a device capable of running iOS 12, so I bought a used one on Letgo.
For $60US I was able to purchase an iPhone 5s with a cracked screen that works just fine. I decided to pickup a new screen from iFixit for another $40US. All in, $100US and I have a great new test device. It actually runs iOS 12 really well.
So, if you’re short on cash, try a used device for testing.