Although there might be some commercially available stocks for the Martini Cadet, I doubt there's much to choose from, so a custom made stock would be the way to go. Unless you really have two left hands, it isn't out of reach for the average person to make his own stock, and you can make it just how you like it, and even choose the type of wood to make it extra unique. Stained Beech is often used for stocks on mass produced rifles and shotguns, as a well stained Beech stock can look a lot like plain Walnut. However, it doesn't have the same qualities as Walnut, which is much harder, stiffer and doesn't bend like Beech often does. If you're on a budget, Beech can be a reasonably good and very affordable source for a stock and looks pretty good when it's stained with a walnut stain like Birchwood Casey Walnut Stain or treated with several coats of an oil like Schaftol Dark or Schaftol Reddish Brown. There are also a variety of exotic wood which have similar properties as Walnut but with a different grain, appearance and a much more unique look, species such as Zebrano, Coromandel, Rio Pallisander, Wengé and so on. They are ofcourse more expensive then Beech, some of them even are even more expensive than an exhibition grade piece of Walnut, but they are definately worth the money because of they're beautiful texture and qualities that make it well suited for a stock, as well as being much rarer than your average Walnut or Beech gun stock. If you want to choose from some less exotic but just as well figured wood, go for quilted Walnut, Maple, or Cherry. They are also more expensive than your average piece of wood, but they have very nicely figured grains on them which make for a very pretty stock.
If you have no prior experience with stock making and plan to use one of the more expensive types of wood for your stock, or even if you're using something ore plain, it's a good idea to buy a piece of cheap Beech to make your first stock out of, to get you an idea what it's like and to practice your technique in case you really have two left hands, so you won't ruin a €200 piece of exotic wood instead. For more info on stock making, there's plenty to find on the internet, so have a look around on google and you'll get a better picture of which techniques to use for drawing your basic shape, carving or milling out the barrel channel and other slots, doing the rough shaping with chisels and coarse sand paper and ending with the final shaping and progresively finer sanding. Good luck!
