Here is the App Store link to the Toybox app:
https://itunes.apple.com/us/app/osmo-numbers-toybox/id1326405743?mt=8

If you are searching from your iPhone, you won't see the app since it's not available for iPhones at this time. And, if you're searching in any App Store other than the U.S., it won't appear either since it's only available in the U.S. at this time.

Note that it's $4.99, and requires the Osmo Numbers game pieces as well as the Osmo iPad base to play.