G.F. Smith's permanent showroom, in Central London. Visits are by appointments only — But I would definitely recommend making the visit. All of their stocks are beautifully archived, along with example publications, and a rotating series of paper installations in both the window & basement, courtesy of Made Thought.
Donlon Books is an atmospheric little bookstore on Broadway Market (in East London.) They sell rare art books, style publications, and homemade photography zines.
Quinto sells second-hand books. I spend a lot of time here and have found some inspiring books over the years, both at Quinto and the other shops on this road. Koenig Books on the corner is good for art books, too.
One of the main sources of inspiration for my work are unusual objects, vintage toys and tools. A great place to find them is this genuine car boot sale, which is clear of 'bargain' toilet paper and questionable cosmetics. Another plus are the comparatively sociable opening hours. (it starts at noon… )