I'm not quite sure where you think the leak is. Pressure testing the system is probably your best bet though. There are a number of common spots which leak against the firewall. You've probably thought of most of them, two possibilities which aren't uncommon but often aren't thought of initially are the banjo behind the spark plug galley and the rear most coolant jacket in the head gasket.
Easter? You need to find a Jewish, Buddhist or atheist auto parts store.
Actually, it may be worth calling around though. Many of the parts stores around me are still open for half the day.