Originally Posted by abfab

You can't get the Oyster card out of a vending machine, but you can top it up at some of the electronic ticket machines.

You can obtain the actual Oyster card from the following places:

At the Oyster website
At most Tube station ticket offices
At over 2,200 Oyster Ticket Stops across London
At some National Rail ticket offices
At eight London Travel Information Centres
By phone on 0845 330 9876†

There is a £3 deposit on the Oyster card, but you can get this back if you no longer need the card.

I beleive the credit on an oyster card has no expiry though, so you can save the card for future visits to London.