Where might one acquire a jingling johnny?
It might help if you told us what one of those is.
A quick google, clicking “Shopping,” reveals…
Here is a good example of a “jingling johnny”:
The are usually not mounted on a platform such as this, and are played by shaking them up and down.
Found an actual one for $250. Is this the sort of thing folks often make?
Unfortunately, those are all small toy models of the real thing.
Right, Chronos was asking what they were (and I didn’t know either). Thought we might get lucky. I see wikipedia redirects to “Turkish Crescent” and gives some other names.
Turkish jingle , Jingling Johnny ,[4] Schellenbaum [5] (Ger.), chapeau chinois [6] or pavillon chinois (Fr.)
In the Ottoman Empire
Follow-up question, then: Where might one acquire a TARDIS? 
It’s not a completely stupid answer: since such instruments are used in mehter music, Turkish music stores would tend to have them
http://www.istanbulmehter.com/index.php/mehter-ekipmanlari/cevgen/cevgen-1-detail.html
and maybe you can find one with an honest reputation that is willing to deliver…
There seems to be many DIY descriptions for them.
OP, a store specializing in percussion instruments may have one for sale, if DIY isn’t your thing. I saw one once at the orchestra; can’t remember the piece it was used for.
If I had to, I could build something in the shape of a guitar…but it wouldn’t be something an actual guitar player would touch with a ten foot pole. An actual Turkish Crescent/Jingling Johnny is a musical instrument beyond my ability to create.
Let’s give the folks in the cafe a chance at this one.
Moving from GQ to CS.
Lark doesn’t list any; how about a stumpf fiddle?
The closest I’ve seen is the lagerphone, that folk bands use. It’s a bunch of bottle caps nailed onto a broom handle.
Jingling Johnny? Isn’t that what guys do at the urinal to keep the last three drops from going down their pants?
I updated the title to make it easier for additional posters to help.