Hey there!
Just to clarify SAE actually implies there are no stamps on the return envelope SASE (Self-addressed stamped envelope) is stamped.

So with all of that said, you can actually send an SAE instead and hope for the best. If you did want to buy stamps, you can purchase them on Amazon or Ebay. You'd want to buy recent forever stamps and look up how many you need to put on the envelope to cover postage. Unfortunately, USPS doesn't allow for international buyers to purchase stamps from them, but I'm in Canada and purchased them from Ebay relatively cheap~

All the best!

Basically, I want to send some fan mail to America but it suggests sending a self addressed envelope. I assume I'd need American stamps? How do I buy them in the UK? Many thanks