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For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat, but for Landsat only you could use Libra as mentioned by @Richard Law): https://earthexplorer.usgs.gov/

For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat, but for Landsat only you could use Libra as mentioned by @Richard Law): https://earthexplorer.usgs.gov/

For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat): https://earthexplorer.usgs.gov/

For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat, but for Landsat only you could use Libra as mentioned by @Richard Law): https://earthexplorer.usgs.gov/

In addition to Landsat, you can also download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI. Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value (NIR-Red)/(NIR+Red).

Primary download site

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

For high spatial resolution, you can download Sentinel-2 data. It has worldwide cover every ten (very soon 5) days. You have 10 meter bands in Red and NIR, so you can compute ten meter NDVI (vs 30 m with Landsat). Make sure that you use the L2A images (radiometrically corrected with SEN2COR) in order to compute a meaningfull NDVI value from the "Top of Canopy" reflectance (NIR-Red)/(NIR+Red).

Primary download site for Sentinel

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

alternative solutions can be found on Seeking Sentinel 2 Images for Nigeria

If you are more interested in the temporal resolution than in the spatial resolution, daily images are available from Sentinel-3 (soon), PROBA-V or MODIS. Again, I recommend using TOA (level2) images.

PROBA-V : http://aida.vgt.vito.be/content/products

MODIS (and also Landsat): https://earthexplorer.usgs.gov/