Anyone know what they're called and where I could find some?

I've seen them in use as height adjusters on aluminum walkers (for the aged, ill and disabled) and collapsible canes. My local pharmacy sells replacement parts for walkers and I suppose you could ask at a drug store near you. if they don't stock them then try a medical supply house, or wheelchair rental place. You can probably get a referral from the drug store.

I can't imagine these things being very expensive. (You could always buy a cane and take one out of it.)

It should be easy to make one, too. Take a 3/16 or 1/4 inch diameter phillips head brass woodscrew. (A 1 inch long to 2 inch long size #10, 12 or 14 should do. Cut the length to about 5/16ths, and use a file to gently round the edges. Drill a 1/16 inch hole in the head of the screw going through the thickness of the head and a slight way into the shaft of the screw. If you have holes in the paddle you should pick a screw which will fit into these holes snugly. If you don't have a hole, then you must drill one. Line up the blades as you like them and drill through both parts of the ferrule at the same time.

You can also use a small bolt, or a rivet. Some of the rivets (like "pop" rivets) have holes in them already, so you just have to cut them down to a short size. Shape the head with a file to whatever size you want. It just needs to be a slight bit wider than the diameter of the shaft so the thing doesn't fall out of the holes in the ferrule.

Aluminum screws would work fine too. Stainless steel may be a great material for corrosion resistance, but it is tougher to drill out, so I'd avoid it here, and stick with something real soft like aluminum copper or brass. Another possibility for an almost ready-made part is a steel "eyelet' obtained from a fabric store. These things are pennies a piece, but usually sold by the hundred. Fill it with a couple drops of hot glue, or epoxy cement to give it a bit more body.

Get a piece of spring wire, and bend it into a "u" shape. Bend the top 1/4 inch of one side of the "u" at 90 degrees, and stick it in the hole in the head of your screw. Shove the assembly into the narrowest diameter part of your ferrule. It should pop out of the hole. When you slide the larger part of the ferrule on top of the smaller one you just push down a bit on the detent and the ferrule goes over it smoothly, and as you rotate it, the detent will pop up through the hole nad lock the ferrule parts together.

Hope this helps.

PGJ