I have no clue where to find the clearness ratios needed for HOMER to give me solar radiation values.
You can enter solar resource data into HOMER by specifying either radiation values or clearness indices. You specify one and HOMER calculates the other. Look up 'solar resource inputs' in the index of the Help system for more information.
The easiest way to obtain solar radiation data is to enter your latitude and longitude in the Solar Resource Inputs window, then click Get Data Via Internet. Look up 'finding data' in the index of the Help system for some other sources of solar data.