A single ticket for one journey in London's zone 1 and 2 is £4. A one day travelcard for use after 0930 mon-fri, any time at weekends/bank holidays is £5.60, before 0930 mon-fri is £7.20. A three day pass is £18.40.
I imagine most online rail ticket sales sites should offer them, but having never used one I couldn't say for definate.
even better then a day out in london for about £20 these days cannot be beatnope, it's £5.60 as the £7.20 fare is BEFORE 0930 mon-fri