Please provide guidance, where could I download this software?
The website that was supposed to hold the software is having database issues which means this could be something temporary and it should come back sooner or later. There is nothing that can be done unless the website comes online. The old domain points to the one that is having database connectivity issues, therefore you can't contact the developers of the software.
Here is the website to try later: http://extension.uidaho.edu/kimberly/tag/reference-evapotranspiration