You can download calculators at Download.CNET.com or at Softonic.com. Both of CNET and Softonic offer a wide variety of calculators, including how they were rated by users and the site itself.
On CNET, to find the downloadable calculators, go to Download.CNET.com and scroll down a little bit on the page. As you scroll down, you will see a list of categories on the left and should see Productivity Software. Click on that and on the next page, locate a Narrow Results section that has a subsection called By Category, with calculators as an option under it. Check that option to find the downloadable calculators. On Softonic, all you need to do is search for calculators using their search bar at the top of the page.