As well as book pages, music and maps I also collect interesting ephemera; letters, booklets, old invoices and receipts. Anything really...
Sometimes I paint directly onto the ephemera or else I scan it, enlarge it to the size I want and print it out on archival heavy weight paper. I then paint my image onto this. In this way I can preserve the original and also play with size and scale to achieve the result I am after in the finished artwork.