To download reviews from one specific source, you should follow the process as described in this article, but with one additional step:
Go to the Data Download tab within Settings,
Select the date range you wish to download reviews from,
Select the source type to download reviews for and hit Download once done.
FYI: If the total count of reviews during the set time period exceeds more than 100, LocalClarity will email you the download link for the reviews as opposed to downloading them straight from within the application.