You can get it from here, https://devcentral.f5.com/s/articles/Microsoft-Powershell-with-iControl.
I copy-pasted the download links below. Right click and save as, for the .msi installers install the .msi then you can grab the iControl.dll file from the install directory, for the zips just extract the folder or open the archive and grab the .dll
Downloads:
v11.00.00 Released August 10, 2013
v11.04.01 Released December 02, 2013
v11.05.00 Released February 18, 2014
v11.06.00 Released August 28, 2014
v12.01.00 Released May 09, 2016
v13.00.00 Released March 21, 2017
v13.01.00 Released November 11, 2017