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I think this has been asked a couple of times.
Asnswer was, well, not many rims do exist in this size with braking surface.
SJS has one but it is steel so braking won't be sharp:
https://www.sjscycles.co.uk/rims-tape/westwood-26-x-1-12-inch-584-rim-chrome-36-hole/HED does one:
https://www.compasscycle.com/shop/components/rims/hed-belgium-plus-650b-rims/And so does Velocity:
https://www.modernbike.com/machined-650b---27.5-rims
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Good god, I'm shocked that anyone is still making steel rims
Edit: it's for rod brakes anyway I think?
The Westwood rim style of cycle rim is used on bicycles with rod brakes, usually the roadster cycles found in Britain up to the 1960s and still common in the South and East Asia and Africa. Westwood rims have rounded sides, so they are not suitable for use with caliper brakes. The inside circumference of the rim has a slightly concave profile to aid with seating of the brake pads
Think he means these: https://www.lfgss.com/comments/14172150/
As they're disk rims they wont do what you need though.
650b rim brake rims are hard to come by. Velocity do the A23 in machined side wall variant.
If you find something cheaper I'd also be interested as I've got a bike I've thought of converting with super long drop brakes, but the cost of the rims has always put me off.