How long can I keep Panna Cotta out of the fridge?
Pack them in a cooler with ice (or freezer packs) and keep them in there as long as you can. Let them be the last thing you put on the buffet... just before the guests arrive.

You could also nestle them in a buffet pan full of ice, so they are displayed in a way that  makes them accessable for guests, but also keeps them from spoiling.