any ideas on where I could get such 7-segment digits?
You should be able to find raw displays through regular vendors like Mouser or Digi-key, or from surplus sellers, but they usually don't have modules with driver circuits (except for digital voltmeters). It'd be easier, and quite possibly cheaper, to buy a used PDA like the Palm that's considered "obsolete" than to make a circuit with drivers. Plus you can do fun stuff like a graphic compass display by writing code for the Palm.
(I just remembered an exception: I did see one module on ebay that I'd bookmarked. Haven't bought one, though, so I don't know how good it is.)