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Are you looking for a shaft collar or a weld on pin boss? A shaft collar normally locks (with set screws or clamping action) to the shaft. A pin boss welds to the base frame with a pin holding the shaft in it using a cross pin, like where the pins go through a loader bucket. If you are making weld on pin bosses a machine shop may have drawn over mandrel tubing, or similar in stock the size you want, that can be cut to the length you need. You might have to ream it out a bit for your fit and drill your own cross pin holes, or have them make the pieces you want.
Here is one outfit that sells DOM stock by the foot.https://mysteelyard.com/index.php?cPath=40&page=3&osCsid=8a265fb1f7c258ef5d52e16e4388a0b1