Good luck finding any extras...these Colibri are super hard to find and anyone that has a good functioning one with extra magazines, they aren't likely to be selling their extras. And you'll probably have even less luck finding them new from a retailer because they stopped making this rifle years ago so any new stock has been sold through long ago.
I went through about 5 or 6 of these rifles in search of the best one, which I kept for a couple of years. It is probably the only rifle to date that I've regretted selling (FYI, I think I had 3 magazines for it and included all three with the rifle).
Good luck...