I know you can get them for the mc21. Just read a post in the classifieds section, seems a set of exhaust hangers are up for grabs "MC21 Garage Sale". Might be what you are after since the MC21 and 28 share the same frame. Could fit, check the parts fiche on here, it's under workshop I think. If the part numbers match, you're on to a winner.
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MC21 Honda NSR250. TYGA 300 conversion.