I think that you buy the CAM from any supplier of the equipment (although I don't know if
Sadoun carries them).
When you subscribe to a service, through a broker who packages several different channels together, they will send you the proper card, and will take care of the authorizations for that card.
This seems to be common in Europe, but I don't think many American viewers use it this way. Some, like
Globecast, simply sell you their own box, with an "embedded" descrambler, so there is no choice of different systems within the one set-top box. Echostar and DirecTV do the same, going so far as to "marry" the box and card, so they cannot work when separated.....you can't put your card in your vacation-home box, for instance, to get the services you have at your regular residence.