Currently I'm dealing with an out of warranty Apple Watch where the screen just went and fell right off, and apparently you cannot buy the glue strips for it unless you buy the whole screen. Apple figured out how to stop people from getting glue!
Here's a tweet from John Gruber of popular Apple blog Daring Fireball, who had the same thing happen to his wife's watch: https://twitter.com/gruber/status/783016045481033729?lang=en
On a side note, iPhones are really easy to get parts for. I had the digitiser fail on a cheap no-name Android tablet, and as far as I can tell you can't get replacement parts for it.
For fun? Or what? I have several relatives who still have iPhone 4 and 5 (and one with a 3!), and none of them have replaced any components. Why would you need to?