Hi Raphael,
You can find the 'GIMMS AVHRR Global NDVI 1/12-degree' data at the link below
http://ecocast.arc.nasa.gov/data/pub/gimms/3g/
The data range is from 1982 to 2012 I believe.
The data is in binary format but I have found a way to read it using R. However, I think you can read it pretty easily with ENVI as well once you get the header figured out.
Let me know if you need some help with this as I've looked into this data a little bit a while back
All the best!
Regards,
Jason