Rostra make aftermarket kits but I don't know if they have one for cars with
'fly by wire' throttles such as I suspect you have.
If you have a bowden cable throttle then a Rostra unit will do the trick.
I have had one on a Peugeot for about 7 years and had no problems at all
with it.

http://www.rostra.com/cruise-control.htm

hth

Bob