I personally fit my carboys with airlocks and age my mead in them rather than fitting a solid bung. With an airlock it is easier for my mead to keep naturally degassing and there is no risk of a carboy popping it's cap and oxidizing my mead. Every once in a while I top up the airlocks with water. I top up above the recommended water level since the mead's inactivity ensures the water would still stay inside the airlock, and this leads to less need of topping up. I can't yet understand why many mazers insist of using vodka in their airlocks to improve sanitation, or to use some liquid which they claim takes longer to evaporate. I just never really had a problem with sanitation or evaporation, especially if you change the water in your airlock instead of simply topping up.
Therefore, my suggestion is that you should buy caps and airlocks unless you already have them
"Shouldn’t we say wine is a mead-like beverage made with grapes substituted for the honey?" - Steve Piatz