I recently commissioned an A3 sized sketchbook with a mix of papers including Fabriano, watercolour and cartridge papers. I was able to ask for spacers to be left so I could make inserts now and then. I was able to choose all sorts of colours and papers but strangely kept it very simple and black! I picked it up today from John Richards, a bookbinder from Leamington Spa and am delighted - can't wait to get started.
Here's some process photos of the book being made. Cutting the different papers to size.