Hi Jana Kleitsch - I personally haven't done any work like this, but it's a common need on Fresh Chalk! Check out a few recent conversations on the topic. freshchalk.com/a/conversati... If it were me, I'd probably take Laura F's rec for Waters Concrete.
If you would be interested in a unique solution you could talk to Rob Leonard at Mathscaper about an artistic concrete form. See mathscaper.com I think they started out doing engraved wood for tables, but Rob told me recently that they are making some concrete forms too.