You can have an idea of the rate here:
www.xe.com
About where is better to change the currency, no clue.
It depends: if it's a "commission free" office, they're using a disadvantageous(for you) exchange rate.
If they're using the official exchange rate, than they have for sure some commission.
My suggestion is to change before to start, or just at the arrival, at the airport, a small amount, like 100/150 $, and then have a look around to see where is the most favourable exchange office
Exchange rate