All Colt 1903/1908 original magazines are expensive, 1908 magazines significantly more so, and factory nickel ones most of all. I would not expect to find one for less than about $200, and that figure may be out of date.
Blued ones would work fine, of course.
I am sure aftermarket 1903/1908 mags exist, although maybe not in nickel, but I don't know anything about their quality. I'd be inclined to think it was mediocre at best, but that's just me.
Ebay is pretty good place to look for magazines, BTW. Good luck!