Try Googling the words 'Great Dane for sale Missouri' each day and see if something comes up. I'm praying for these two dogs and hope that they will be found soon and that no harm comes to them.
I'm sure they've done this but make sure they contact every shelter and vet clinic in the area so they can all keep an eye out.
Posting an ad in the newspaper will really help and if they can afford to place a display ad rather than one in the classifieds it will get more attention.

Contact the local news crew and get them to run a story about it. You'll get thousands of people to keep an eye out that way!