A lot of rescue centres have adult cats who are "indoor cats" - mine was when I got him (he was actually agrophobic!).
It really is worth looking - try the Cats Protection League, the RSPCA and look for independently run rescue places in your area. You'll be surprised how many cats are indoor only. A place I help out at has loads and loads of them - in case you're in South East London, Kent or Sussex it's the Celia Hammond Animal Trust www.celiahammond.org and they have branches in Lewisham, Canning Town, Hastings and Tunbridge Wells. They often look for indoor only homes for a variety of reasons.
Good luck.