There is a sort of combined kiosk/supermarket upstairs at the main station and some bakery shops etc. downstairs. But considering the numerous drug addicts loitering in the underground mall I'd give it a miss.

Now, if you like German meat and sausages, then this is the top place in Frankfurt. I have even seen some shops in other German cities carrying their wurst.

http://www.gref-voelsings.de/

You can find their location and opening times under "Kontakt". Open Mon - Sat from 7am, it seems.