The answer is the same for when exchanging any 'home' currency (where you have a bank account) into the currency of the country you're visiting:

Withdraw it from the ATM, with a debit card.

As long as you withdraw reasonably large amounts (as a rough guideline, >$200), this is virtually guaranteed to give you the best all-in rates.

See What is the best method for exchanging dollars for Euros? for a fuller discussion.

The answer is the same for when exchanging any 'home' currency (where you have a bank account) into the currency of the country you're visiting:

Withdraw it from the ATM, with a debit card.

As long as you withdraw reasonably large amounts (as a rough guideline, >$200), and you're using a bank that doesn't completely screw you over on their own fees, this is virtually guaranteed to give you the best all-in rates.

See What is the best method for exchanging dollars for Euros? for a fuller discussion.