Not quite the answer to your question, but have you considered a Halifax clarity credit card?
Take the money out of the local ATM in cash, no charges and a good exchange rate.
You do pay a small interest but more than covers the better exchange rate.
You cannot pre load the card, and need to tell them where your going in advance so the card doesn't get blocked.
I haven't used in in US but have used in Europe. I went on my last holiday with virtually no local currency, bit scary but it was fine.
Oh and if you do decide to use this, apply in branch there are benefits for doing so.
I don't work for, or have no reason to "plug" this card. It's just an alternative to consider and please don't blame me if you end up borrowing off others on the tour if the card gets swallowed!
I heard about it via the moneysavingsexpert site, plus a relative recommended.