I've replaced a couple on my Panda in the past - I think they had fallen off (rather than been nicked).
IIRC they were about £8 each from a Fiat dealer. They were a couple of different designs available at the time, and probably more now, so worth making sure they have all the options in stock to save a wasted journey.
There's a bit of a knack to fitting them, which I think involved gently pulling apart the metal ring that holds the plastic lugs in place while you are putting it on the wheel (it was a while ago that I did it). If you do that, you don't need much force to get it on the wheel - I think there's a risk of the plastic lugs breaking if you were to just bash it on with a mallet, which I suspect is why I lost two in the first place.