for the magazine. They list four, all in .25 caliber. (There wouldn't be much point in making 5mm magazines as the ammo is unobtainable.)
I have no source suggestion on the grips. You could probably make them or have them made but of course wouldn't look original (unless you can do a real good job of carving plastic).
You should be able to tell .25 from 5mm just by looking at the muzzle, or use a ruler. If the hole is about 1/4 inch (.25") it is .25 caliber. If it looks a lot smaller, it is 5mm (.18 caliber), or about 3/16th on your ruler.