2. Off-base, the credit card companies have the best exchange rates since they are monitoring these currencies by the minute. When you arrive at your final destination's airport, find an automatic teller and withdraw some local currency using your credit card (you will need to know your PIN). Then walk to the nearest shop and buy a Coke or something else small with a big bill of local money. Now you have smaller denominations with which to pay the cab driver, porter's tip, etc. Don't get caught paying them with a big denomination and remember than in some places tipping with American dollars is illegal.
3. To plan ahead for your currency needs, consult Yahoo's Currency Converter.