Maillard sprocket: is really this rare? Due to (understandably) large wear after 16 years of usage, I wanted to replace sprocket, chainwheel and chain on my byke.

With my surprise the workshop where I took my byke was not able to find a spare for the sprocket, a 6 speed Maillard, and told that in the Netherlands they are pretty rare. They tried to install a 6 speed Shimano but it did not fit.

I did also a quick search on some spare parts dealers in Italy, and also they didn't have it.

Is it really that rare, or I have been simply out of luck?
If the Shimano freewheel didn't fit, it means that your bike uses French threading for the freewheel (which isn't surprising given that it has a Maillard freewheel; the other non-bmx standards are interchangable). You're basically out of luck and need a new wheel (or at least new hub). 

Depending on the hub, you may have to re-space the frame(*), since 6 speed likely means 126 mm rear spacing, where a newer hub would be 130 or 135 mm. This isn't difficult, and a shop that has a decent amount of clients with older bikes should be able to do this easily. You can also still find 126 mm wheels on the market if you look (Velo Orange still sells them, for example), so you wouldn't need to respace the frame. Or, you can respace it yourself. But this is the way I'd go. 

(*) Assuming its steel, which given the age, it probably is.