There are a handful of gardening sites that sell the egg cases, called ooths. I got mine from Hirt's Gardens - needed to get a native mantis type so as not to introduce a non-native species to my area.
Got them specifically for natural pest control. Hopefully they come back next year like the sunflowers did - guess we'll see!
I clicked thinking the headline sounded like something written by a low-ambition mad scientist raising a mantid army to conquer the local insect ecology. Imagine my surprise when I was right!
Somehow, your post has made me really see them as a predator for the first time in my life, and picturing myself on the wrong end of a buffalo-sized mantis has left me terrified.
They're always such cute little buggers, and I love finding them. Easily among my top 3 favorite insects.