The most convenient places to buy an Oyster card are Underground stations across London, including Heathrow airport. You can buy one using either cash or your credit/debit card. Although there are a few manned ticket offices in London, the likelihood is that you will buy an Oyster card from a ticket machine.
Is Visitor Oyster card better than Oyster card?
In conclusion, the main difference between Standard and Visitor Oyster cards is that Standard Oyster cards are reusable, while Visitor Oyster cards are disposable. In addition, standard Oyster cards do not come with a daily price cap, while Visitor Oyster cards do.