It is not always possible, or even desirable, to purchase local currency in advance of an expedition, but dollars are acceptable almost anywhere. Unless you know differently this is the currency that you should take with you on an expedition.
Most major international airports now have ATMs where you can get cash on arrival, but beware this is often provided in high denomination notes - not what you want for tips and small transactions. ATMs may be difficult to locate once you leave the airport and are certainly not guaranteed in small towns and villages. Ensure that you have adequate local currency before leaving for the mountains. Remember that tipping is an established way of life in many countries and you should have adequate small denomination funds for this. Getting small denomination notes is not always easy, but hotels and banks will often break down larger notes. Exchange bureaus exist in cities and larger towns but usually they will only change currency; not credit or debit cards.
It is usually cheaper to use debit rather than credit cards, but this may depend on your bank. Remember to alert your bank or building society to the fact that you are likely to be making transactions overseas and tell them which country this will be – having your card blocked while on an expedition can be more than simply an annoyance, it can be disastrous.
If you are taking local currency from the UK try to get ‘clean’ notes. Damaged and heavily marked notes are often not accepted, even in the USA.