I only had two minutes left to catch the bus so I didn't think I'd make it, so I went in to Staples, an office supply store which is right next door.
I asked in the technical department whether they had AC adaptors but they didn't. The salesman suggested Komet (who said they had none), then Karstadt (who had sent me to Komet to have a look there), and finally eBay (which only had one original adaptor when I looked, which went for quite a bit of money to someone who bid against me in the last half-minute -- and I wasn't that keen on buying third-party stuff sight unseen).
Then he called another salesman over and asked him whether he knew any help. He asked me what kind of Jornada I had and when I said "a 548", he said "Boss, I think someone called you over there" and told me to stop aside.
Then he said he used to have a Jornada 548 and still had a travellor's AC adaptor, a car adaptor, and a third-party case as well as a "PDA pen", would I be interested? The adaptors are original HP, he said, which sounded good. I said maybe and he went to see when he'd next be working and we agreed on next Thursday: he'll bring in his stuff and I can try it out in the shop to see whether it fits and then "we can talk about the price, then we'll both have a good laugh and then settle on a different price -- or something like that".
We shall see.