How can I send an SASE abroad with a letter from the UK?

I am in the UK, and wanting to send a letter to the USA with a SASE. Bit confused as to how I can go about doing this, as I obviously can t use the UK stamps on the envelope. Do I have to somehow order USA stamps to use, or is there another way. Can t seem to find an answer on the Royal Mail website

Anonymous2019-11-13T20:47:34Z

Interesting question.

You can purchase and print postage at the USA post office website (usps.com).   You could then mail the pre-paid shipping label in your UK letter which the USA recipient could then affix to a USPS flat international envelope mailer. 

Hopefully someone will come along with a less cumbersome idea...