You can only buy up to 4 days in advance IIRC. I suppose you could just
about get a friend/colleague in London to buy you one and post it to you
(if you trust the Royal Mail...)

It is seven days for the one day and weekend travelcards, the four days is
for weeklys and longer. So even more time.

I would have thought the FastTicket machines at Kings Cross do one day
travelcards though and there must be about eight of them in the booking
hall.