[abel29a]

If you look at Cadiz on the map and there is a blue ship name there you can indeed dock and get resupplied. If you hit F1 and bring up the help screen one of the screens will list all the supply ships and milk cows and their availability dates.

So far I think all the messages I've recieved via radio about resupply ships have corresponded with the arrival of actual supply ships in-game, so chances are good.

Remember tough - resupplying at such a ship is just like going to a base - you'll get repaired and new torpedoes, and sometimes you'll spend a long time there if you have damage - which sort of ruins the immersion. In addition, you should complete the mission objective first, or you'll lose out on the renown for completing the mission, because when you've docked at the supply ship you'll start a new mission with destionation NULL.