Your best bet is probably eBay, especially if you don't mind 'unofficial' recordings. I've seen The Admirable Chrichton on there recently (it's still there, actually), both under its proper title and also under the title Paradise Lagoon! There's also been Quality Street in the past, and I expect there have been others. The Little Minister (Katherine Hepburn's version) was officially released in the USA on Laserdisc, and a copy of this is currently available on eBay - but who's got a Laserdisc player these days???