Here are a few places of interest!!
A great friend of mine in London is Iain Michael Brunt who is the owner of Pairs Antiques Ltd. Which, as you can gather, is a specialty shop dealing in pairs of Antiques!! If you want something in "twos", Pairs is the place!!
THE source for fine, authoritative books on Art, Artists, Gardening and just about everything in the world of Fine and Decorative Arts is the Antique Collectors' Club. You can order items direct from their US website and the service is super.