You buy one day travelcards at any tube station ticket machine. To safeguard innocent travellers, the machines also issue travelcards automatically if you try to get a return ticket in off peak hours of travel. Don't know how familiar you are with it, so I 'll add that travelling Russell Square to Oxford Circus doesn't require two tickets; going up to buy another one at King's Cross is not required; even though you change lines at King's Cross (just as well, because the queues at King's Cross, both for the machines and the ticket windows there, are horrendous at most times!). You simply go from one line to the other. The ticket issued will be valid for any journey in the central zone. The travelcard you'll need is likewise one which is valid for a day's journeys in that zone, more than big enough for normal sightseeing.