NDVI is easy to calculate from a wide range of different image sources. In terms of already prepared NDVI data, MODIS data are processed into several different vegetation indices and made available on 16-day, monthly, and yearly intervals at different resolutions. MODIS NDVI data can be downloaded from NASA's Land Processes Distributed Actice Archive Center or from the Global Land Cover Facility.
Prepared MODIS and AVHRR NDVI datasets are also used in online tools like RangeView. While these NDVI datasets cannot be downloaded directly, they can be viewed and used for simple analyses via the website.