A simple way to buy euros for travel before leaving the country is to purchase them from a bank. Many banks provide this service. Expect to pay fees based on whether you are a customer, the amount of euros you're buying, and whether you want the funds delivered to you.
Prepurchasing currency before travel is a potentially risky move, as physical currency can be lost or stolen when you are on the move, and it cannot be replaced if this happens. One way to get around this is to purchase only enough for your initial needs when traveling and to use credit cards and ATMs in the country you are visiting. Purchasing currency from a local bank in the country you are visiting is also a cost-effective option.
Credit cards and withdrawing cash from an ATM work throughout most of Europe, and the fees charged by your financial providers are likely to be comparable to the fees you might pay for currency exchange. Some European countries insist on credit cards offering a Chip-and-PIN facility, but ATM cards almost always work. Check with your bank regarding its its fees and limitations, and remember that fees are generally charged per transaction, so a single large withdrawal costs less than several small transactions.