If it's an indoor surface + backing onto other furniture (rather than an external or internal wall), then I'd go for MDF/wood too rather than plasterboard. The latter can be a little fidly, especially if using a framework to hold the board and joins that need to be taped or filled (or both, depending on the board) and can crack/break if not handled correctly.
Alternatively you could "clad" the back of the units with wood, e.g.
(first photo found on internet)
In terms of where: Jumbo, Obi, COOP Bau & Hobby.
If you are buying a lot of material, a trip to the border and somewhere like Bauhaus or Hornbacher (sp?) would be worth it, as the materials are cheaper and in effect you also get a 20% discount (VAT reduction) if you buy less than €260 of material per person in the car (i.e. you can buy €520 if two of you are in the car).