How do I go about Acquiring £5 notes?

How would I go about acquiring a what I deem to be a large amount of £5 notes. So it’s my sons 13th birthday in 2 weeks time and I have no idea what to buy him. So I was thinking of doing the money in a cake idea where they pull out a long stream of notes so I need to get my hands on 100 £5 notes. How would I go about this ?