In my experience shops will exchange without a receipt if goods are faulty or damaged. They might ask if you have the receipt but if you just say that you couldn't find it, I think they'd be unlikely to quibble over a straight exchange.
I bought a copy of the lovely book The Jolly Postman by Janet & Allan Ahlberg for my 3 year old and unfortunately it had two copies of Goldilocks' letter and no witch's letter. It was sealed when I bought it from Waterstones and I didn't keep the receipt. Looking at the publishers website it says faulty books should be returned to the shop but I'm not sure I'll get anywhere without proof of purchase. Also it's not faulty as such just missing a bit! Does anyone have any suggestions? Thank you!