Dear Nadia

You can derive it from Landsat images

http://www.mdpi.com/2072-4292/7/3/2352/remotesensing-07-02352.pdf

http://www.mdpi.com/2072-4292/8/1/38/pdf

Dear try these websites good luck hopefully will be very useful for you

http://www.esa-soilmoisture-cci.org/node/145

http://www.esa-soilmoisture-cci.org/dataregistration

https://www.esa.int/Our_Activities/Observing_the_Earth/Space_for_our_climate/Nearly_four_decades_of_soil_moisture_data_now_available

https://ismn.geo.tuwien.ac.at/data-access/

http://fght.utm.my/kasturidevikanniah/2017/07/25/soil-moisture-and-soil-temperature-free-data/

my regards

Thank you Dr. Hayder