If you are buying them with cash, get them here. Changing �� in the US is next to impossible outside tourist areas and large cities and poor value in those areas.
But getting them from an ATM with your debit card is usually the best way.
i would recommend taking some US$ with you but if you have free banking in UK then use your cash machine in any branch of HSBC, your withdrawel will be converted into � via the business rate meaning an extra 9cents per �, it all adds up
get them there from an ATM would be my advice, but watchout for the fees - usually $3 added by your UK bank.
Nationwide and the post office offer credit cards that have no foreign exchange fees added.
I used a Post office one in Florida last week, and got $1.98 to the �.
A cracking rate, as long as you remember to pay the bill when you get back!