You can buy 'em at a lot of places.
Here's a few:
www.egauges.com
www.ravspec.com
www.phase2motortrend.com
www.hopupracing.com
just to name a few.
Almost all performance shops should carry them.
*EDIT* Just noticed you live in France. I don't know if the above named shops ship internationally, but it's worth a try.