What can you substitute for port wine in a sauce?

A sherry could be used, although sherries are usually drier than ports, but equal in alcohol content, so you might want to add a bit of sugar to make up the difference. Otherwise you could use any heavy-bodied wine, but might need to add a bit of cognac or brandy (both are distilled wines) to boost the alcohol content, since both sherry and port are fortified (high in alcohol).