The tubing I've seen, solid rods I've not. Found a place that sells .57" [14.8mm] tubing by the foot, 23 gauge [0.023"] wall thickness at bottom of this page at Sante Fe Jewelers Supply. Not sure that helps, I just Googled "Silver Tubing" and came up with a few Sites that sell it, including that one. Thinking of making a silver pen, perhaps a repair? You've got me curious now.
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"None of us can have as many virtues as the fountain-pen, or half its cussedness; but we can try." ~Mark Twain~