You don't need a train ticket, the offers are equally valid if yoyu buy a London Travelcard at a National Rail booking office rather than a Tube station.

So in central London you can buy at Victoria, Waterloo, Charing Cross, London Bridge, Liverpool Street, Kings Cross, St Pancras, Euston, Marylebone and Paddington as well as smaller stations like West Brompton, Kensington Olympia, Cannon Street, Moorgate, Fenchurch Street, City, Farringdon and Old Street okus a few others that I can't think of straight away