If you've got a contactless card you can just use that, costs the same as oyster. Or as you suggest you can get an Oyster card from a station or shop that does top-ups.
Since they went to contactless I've stopped using my Oyster card,
Yep, contactless bank card all the way. If you're not familiar with the system, you need to touch in and out on trains/tube *even if there is no barrier* but only touch once on buses as it's a flat rate per journey.
It has to be a London train so if you were travelling on a train that took you out of London you couldn't have used Oyster or contactless. On the tube and trains within London you touch in when you board the train and touch out at the destination so it knows the correct charge (otherwise it will just default to the maximum daily fare).
In reply to Pedro50: That is why you have to touch in and out.
When you go further than London you may need a ticket as well or instead. Worth knowing that if you want to go to some places just outside London you can save quite a bit if you can be bothered to get off the train at the last oyster station, touch out and then have a ticket from there.
Did this for Gatwick but apparently thats now got oyster.
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