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 https://gis.stackexchange.com/questions/190681/where-how-to-download-sentinel-2-images