These are the system requirements for this application. No, you can't purchase it on its own because it's bundled with a package.
System requirements:
- Windows Vista®, Windows 7, Windows 8
- Adobe Photoshop CS4 through CC 2015
- Adobe Photoshop Elements 9 through 13 (apart from HDR Efex Pro 2, which is not compatible with Photoshop Elements)
- Adobe Photoshop Lightroom 3 through 6/CC
- Mac® OS X 10.7.5 through 10.10
- Adobe Photoshop CS4 (CS5 for HDR Efex Pro 2) through CC 2015
- Adobe Photoshop Elements 9 through 13 (apart from HDR Efex Pro 2, which is not compatible with Photoshop Elements)
- Adobe Photoshop Lightroom 3 through 6/CC
- Apple® Aperture® 3.1 or later
For additional information, I recommend checking the following website: https://www.google.com/nikcollection/products/silver-efex-pro/#