You can get them at Heathrow. Pay a £5 fee and add as much on it as you like. Depending on how many zones through which you travel ( most of the tourist sites are within one zone), there is a maximum daily cap set at £6.70. You can get a refund for unused amount £10 and under. The card can be used in the Tube, buses, and some river travel.

Routes run in reverse. Take the Central Line, at Shepherd's Bush to Ealing Broadway. Transfer to the Piccadilly Line to Heathrow.

Across the Thames, from the Tower, is the Tate Modern and the Globe Theatre (take a water taxi to get across). Directly opposite the Tate, is At Paul. Walk across the Millennium Bridge to get there

Have fun.