Ours comes free with our CC and is a fantastic policy but that's no help to you Do make sure you chose a company that provides a minimum of £10,000,000 Medical cover - hospital treatment in the USA is astronomical.
I bought ours through M&S (which i think is AXA) we took out annual insurance for a couple and it was £110 total. and had that cover of medical as previous post.
Ours comes free with our bank current account. Check what your bank and credit card companies provide first - you might not need to buy anything.
If you do need to buy a policy, you'll likely find an annual one is as cheap as one just to cover the period of your holiday, especially if you're going for more than 2 weeks.
Places like Boots, banks, travel agents, building societies and of course insurance brokers all sell travel insurance. Prices and cover vary enormously so shop around a few reputable sources.
When shopping around do compare the cover they offer as that can vary even more than the price.
Thanks for all the different advice, think i'm going to go with annual insurance so it'll cover next years holiday as well although not decided who with yet.
Thanks everyone, In the end I have bought annual worldwide couple insurance from Club direct through Money Supermarket for £62.30 which includes the £10 million medical.