Usually Travelex or M&S (yes Marks & Sparks!), bear in mind that buying on line with a credit or debit card is classed as a "cash advance" so is usually liable for about a 2% commission charge plus interest straight away. Usually you can look on line and then troll round the Post Office, Barclays etc. and see who's the nearest, usually they're within 2c of each other so not a huge difference.

If you can open a US bank account (Citibank in the UK do joint currency accounts) you can get the commercial rate when transferring between accounts but ask about charges as some banks are quite expensive.