You can download and purchase the last compatible version of iReal Pro directly from us.
Mac OS X 10.6 - 10.12 (Blues and Salsa in-app styles are not available for purchase in this version):
Download the app from here:

Note: updates won't be available for macOS 10.12 and lower. In case you upgrade to a newer operating system, email us directly with the receipt of your purchase to get a code to the current version of the app.
If you have a computer with an older OS X, email us the receipt of your purchase and we'll send you a code to a compatible version of iReal Pro.