Sometimes this seems to be on a turn-by-turn basis - ie 1 turn some combat supply can be drawn from a nearby port - the next it can't - it has to go to Naples which might be 20 hexes away & I don't understand why that is the case.
2/ On the reinforcement screen when you click on a unit there is a "Combat supply" button - this appears to give the unit 1 level of combat supply before it arrives, duly deducted from the available total.
It's quite useful for units you want to get into action ASAP - but it isn't documented in the manual as far as I can see apart from a slightly cryptic comment on page 16 -
At the start of the Italian campaign, all units have a low amount of combat supply. The first thing a player will do is add combat supply to some or all of his on-map forces.
IMO it probably should be more explicit and in the reinforcements section
3/ Page 32 of the manual says "[Axis]Reinforcements enter at Verona." - but not all of them do.