San Francisco is the major financial center for US-Pacific trade and the many bank branches will exchange your Euros. There's an exchange booth in one of the largest downtown shopping malls (other locations mentioned; I am not affiliated with that link). The problem is that exchange rates are terrible everywhere in the United States. I don't even get a good rate as a relatively high net-worth customer on wire transactions at my regular bank, much less as a tourist walking up.
Your best bet is using your own debit card in an ATM. California used to prohibit ATM fees for ex-USA credit cards. This is no longer true, but some banks will not charge you anyway. (What your home bank does is up to them.) Your second-best bet, depending on risk tolerance for pickpockets, is to get USD in your own country.