Go to the tube stop. Face the turnstiles.
Turn 180 degrees. There's a ticket window
down a very short corridor. They may not be open if you get in early.

You might want to note that the regular zone 1-2 farecard won't get you into or out of Heathrow; you have to pay a supplement. Also, they aren't good before 9:30 am. However, the cards available from US travel agents are not subject to those rules, as I recall (you get a voucher from the agency, which you redeem at this office).

I have before me Les moyens de transport a
Londres, 1999 edition, which gives the following fares:

day card, zone 1-2 L3.80
day card, zone 1-6 L4.50
good for the tube and regular buses

weekend card, zone 1-2 L5.70
weekend card, zone 1-6 L6.70

weekly card, zone 1-2 L17.60
weekly card, zone 1-6 L34.90
Weekly card require an LT-issued photo ID; this is not required of weekly visitor cards (the ones available from travel agents).

day LT card, zone 1-2 L4.80
day LT card, zone 1-6 L7.50
good for the tube and all LT buses plus the Docklands railway