If Numrich doesn't have them, these guys do...
https://www.buymilsurp.com/magazine-42a-cz50-cz70-czech-pistol-new-floorplate-p-366.html
Also, check eBay.
https://www.buymilsurp.com/magazine-42a-cz50-cz70-czech-pistol-new-floorplate-p-366.html
Also, check eBay.