Tomorrow (5/29/2015) the Silver Mario Amiibo will be available at multiple stores, but right now I've been informed that you can ONLY preorder this special Silver covered Mario Amiibo from Target on a mobile device. For some reason, he same option to Pre-order the Amiibo isn't available on a PC. My informant to Amiibo information has told me that he believes that the ability to Pre-order this figure from a mobile device won't last long, especially with the release just around the corner. On the Target listing for the Amiibo it says "Sorry that item is no longer available."
So it would seem that my informant was correct on the pre-order ability coming to an end soon, but it was most likely because the amiibo was sold out of pre-orders.
Source: Target site Game on and Keep it Rehl!