Landsat 8 and 7 have a 15 meter panchromatic band which can be pan sharpened with other bands or used individually. ASTER with 15 m VIS-NIR bands is another option. EO-1's (now decommissioned) Advanced Land Imager (ALI) has a 10 m panchromatic band and has free satellite images over some parts of Sri Lanka [Source: NASA & USGS]. The Sentinel family of satellites and sensors such as Sentinel -1 SAR and Sentinel - 2 (Multi spectral Imager) with 10 meter spatial resolution provided by the European Space Agency [ESA] is also a very good alternative. All these satellite data sources that I've mentioned are free and very popular. You may find more relevant data if you explore the first link that I've attached.

https://earthexplorer.usgs.gov/

http://reverb.echo.nasa.gov/

http://www.esa.int/Our_Activities/Observing_the_Earth/Copernicus/Overview4

Ob Earth Exploret (https://earthexplorer.usgs.gov/) and ESA Sentinels Scientific Data Hub (https://scihub.copernicus.eu/dhus/) you can find Sentinel 2 data with very good spatial and temporal resolution.

http://gisgeography.com/free-satellite-imagery-data-list/

https://earthexplorer.usgs.gov/

https://reverb.echo.nasa.gov/reverb/#utf8=%E2%9C%93&spatial_map=satellite&spatial_type=rectangle

http://www.dgi.inpe.br/CDSR/

https://scihub.copernicus.eu/dhus/#/home

https://www.class.ncdc.noaa.gov/saa/products/user_profile

https://earth.esa.int/web/guest/eoli

http://gisgeography.com/usgs-earth-explorer-download-free-landsat-imagery/

Thanks very much all for your valuable information 

If you are the researcher or the student of some University, there is an option to apply for high resolution satellite data but only one scene. Therefore you have to select exact study area where do you want to conduct your research. You can check Digital Globe website they are helping the students.