Like I did last year I made some change before I left. It prevents you from the enormous fees you have to pay when you take money from an ATM or at the airport (around 10% I think).
Since I had no problems last year I used the website http://www.multi-change.com/ again. If you are in Paris you can go directly to an agency because they always have crowns available. The website is handy if you monitor the rates, then you can order when it seems most advantageous. I chose the delivery in agency, to save time, expense, and anxiety because of the Post (even if I never heard of problems about it, and you should also know that as of a certain amount you have to go get the envelope to the post office with a proof of identity). To fight against fraud you must now send an email with a scan of your ID and proof of residence (you won’t have to do it again the next times). When you get your order in an agency it is usually ready within 24 hours.