Specifically 28mm ± .1mm according to FIG Apparatus Norms: http://www.fig-gymnastics.com/publicdir/rules/files/en_Apparatus%20Norms.pdf
Alternately! If you know where to get a ~1m length of 28mm steel bar for cheap that would be cool too. Knurled/textured or smooth is fine.
McMaster has like 30mm and 25mm or something, or maybe 1-1/8in which is close but no cigar
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