Unfortunately, that isn't possible. You can only search Inbox by student name. You can't search for content. See the discussion linked below.
It's important to understand that Inbox is a messaging tool, not an email client. If you have been getting Inbox message notifications to your email, you could try searching your emails.
A tip, for messages that I know I am going to send out repeatedly, or every semester, I first type them in a wordpressor or notepad app and save them. Then I copy/paste into a Canvas message or announcement.