Can you purchase international stamps in advance?

A friend of mine has joined the Peace Corps and just found out that she will be placed in Indonesia. For a going away present I want to get her fancy stationary along with enough postage to send letters as often as she likes. Is there a way that I could purchase some form of international postage that would send her letters from Indonesia to the US before she actually leaves the states?

Alternatively, what would be a good going away gift for someone that is about to be out of the country for 2 years teaching English to Indonesian children?