Gregorio - You haven't mentioned if you have ruled out other options, so here's what we do if we need cash up front in a new currency.
We have contacted the proprietor of the accomodation and have asked if they will take installments over a few days. Some have held our Credit Card number as a 'deposit' (even though they don't take credit cards) and let us pay as we could withdraw money.
We have also recently taken ATM cards for different accounts to get more money - such as 3 separate accounts at different institutions with $500 daily ATM limit each. There are online banks such as Captial One Direct banking and other credit unions that have really low or no ATM fees. Even taking an ATM card from a bank with higher fees can be cheaper than exchanging cash at a bank. Now, our Capital One and Wells Fargo ATM cards are only used if needed in Europe. I just test them once before we go and notify them of what countries they will be used in.
I have also found, specifically in Amsterdam and Bruges a couple of years ago, that even though I had a $500 daily limit the ATMs wouldn't let me take the full amount. I had to make multiple withdrawals at different ATMs to hit the Euro equivalent of $500.