Ampersand Books is the main UK website for 'dimensional and interactive books.'Megan and Michael Dawson are real fans of pop-ups etc and their website is a delight with animations showing books popping up and even a quiz.
Stella and Rose's Books sell rare and out of print books. Specialising in Children's Books they have over 50,000 books in stock at their two sites at Tintern and Hay-on-Wye. Join their database and they will regularly e-mail a catalogue on your speciality.
Abe Books. This a marketplace specifically for books, linking buyers and book-sellers. Bookshops download their stock-lists and you can search them by author,title or keyword (pop-up?).It is Canada-based but with lots of UK suppliers.