That's if you buy direct from TfL or online - there is no way they could (or would want to)
calculate the discount.

http://www.tfl.gov.uk/tfl/fares-tick...fares-2005.pdf

Page 26 of this lists the NR stations selling Oyster Cards:

Barking, Beckenham Junction, City Thameslink, Drayton Park, Ealing Broadway,
East Croydon, Essex Road, Euston, Fenchurch Street, Finsbury Park, Greenwich,
Gunnersbury, Harlesden, Harrow & Wealdstone, Kensal Green, Kensington Olympia,
Kenton, Kew Gardens, Kings Cross (Thameslink), Lewisham, Limehouse,
Liverpool Street, Marylebone, Mitcham Junction, New Cross, New Cross Gate,
North Wembley, Queen's Park, Richmond, South Kenton, Stonebridge Park,
Stratford, Upminster, Walthamstow, Wembley Central, Willesden Junction, Wimbledon

Most (all?) of these are tube/tram/DLR too! Funny that

Yes. There isn't much incentive for say SET to start selling Oyster Cards at Bromley South.