I've just booked 3 weeks in the US, it's been about 4 years since I've been & have always bought travellers cheques in the past.
I see there are many more ways now to buy currency, with prepaid travel cards & such like, so what is the best way to buy these days & where from?

M&S are usually very good.

Travelex, but pre order or get a Nationwide debit account and use your card like you do at home withdraw via ATM or as a debit card.

Nationwide now pass on the Visa fee (0.84%) for ATM withdrawels in the US (IIRC, it's anywhere outside Europe) now:

http://www.nationwide.co.uk/current_account/foreigntransactions.htm

They don't charge a commission fee though. We have a Nationwide account and will be using it when we go to Boston in November. I think in Canada we found a couple of ATMs that didn't like the Visa Debit card but no problems when we were in NYC.

If you don't have an account already, then MoneySavingExpert says the Abbey Zero card/account is entirely fee-free overseas (but charges interest):

http://www.moneysavingexpert.com/travel/cheap-travel-money#topcards

MSE is probably a good place to see what will work best for you.

I have a FairFX US$ card that I load up when needed – good rates [currently 1.6350], $2 charge at ATMs, no charge when used as a debit card

Given the very good rate – I find it works out cheaper as I only use an ATM every couple of days or so

http://www.fairfx.com/

Nationwide now pass on the Visa fee (0.84%) for ATM withdrawels in the US (IIRC, it's anywhere outside Europe) now:

True, but their better exchange rate cancels that (0.84%) out, never had a problem using card in USA.