Money Exchange World has been my go-to for buying USD for the last five years; their service is fast, efficient and courteous. I really like that their daily rates are posted online so you always know what you're getting. I've previously shopped their rates and they've always beat other money exchangers and are much better than any big 5 bank. They carry a wide variety of currencies and you'll get rates better than those posted by changing larger amounts (greater than $3k from my experience). I won't go anywhere else to buy USD.