How do I change czech koruna in prague with good rates?

Am going to Prague in April and found out that money changers in my country do not sell czech koruna. This means that I can only change money in Prague itself. I have read alot of stories where illegal money changers and legal money changers take that alot of commission from the transactions and giving you outdated old currencies instead. Are there any money changers in Prague where I can get good rates for my country currency? I also heard that it is best to take out cash from ATMs instead as the commission is not that much and much more reliable? Is there any limitations to how much I can withdraw per transaction in Prague ATMs?