You need the stalk which can be had off ebay - the prices range from £10 to £50+, Search for either Citroen or Peugeot - I picked mine up for £15 and was from a Peugeot 206
There are two types. One is just Cruise Control and the other is CruiseContol/Speed Limiter. Both will work, but without a change to you accelerator peddle (£100) the Speed Limiter function won't work on the second one
Looks like this
Then you need to get it activated either by a main dealer, or if you have access to a Lexia/Diagbox you can do it yourself and save a few quid
Here is my guide on how to do it and Citroen's original documentation