Flash Paper can only be sent by land (truck), it cannot be shipped by air. They pack the paper in a liquid, to prevent it from igniting. Upon arrival, it has be left to dry out. Again, that is a several day process.
I really don't think there is any magic shops in Mexico, I have never heard of any.
The best thing to do is check with shippers to see if they can advise you on rules of law sending flammable material across the border. If they can even do it, you are looking at least a week or more shipping time.