OP was ordering US stamps to include _in_ the letter, on an SAE (self-addressed envelope) they were sending _from_ the UK, so that the FSF could reply (from the US) using said stamps.
As a millennial myself, I have no idea where else I'd look for <recipient country> stamps should I want to include them on a SAE I was sending to said country, so that they recipient wouldn't incur the cost of replying to me.
I don't find looking on eBay particularly strange, though I'd do a quick search for alternatives first.