Combustion Industries has several sissy bars (sold on etsy.com)
(above photo) Top Valuable Sissy Bar Backrest With Comfortable Pad And Luggage Rack Fit For Honda Rebel 300 Rebel 500 CMX300 CMX500 2017-2021 (Black) https://a.co/d/iUBuuxW
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Givi sissy bar (short):
https://fortnine.ca/en/givi-sissybar-with-luggage-carrier-ts1160b
Worldmotop Motorcycle Sissy Bar Backrest Rear Passenger Seat for Honda CMX 500 Rebel CMX 300 CMX500 2017 2018 2019 2020 2021Detachable Backrest (black) https://a.co/d/aCUlDg0
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https://www.aliexpress.com/item/3256802811559088.html?gatewayAdapt=4itemAdapt
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https://www.accessoryinternational.com 
https://www.accessoryinternational.com 
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Acc-Creativity sissy bar on Amazon $169.90 USD
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Angu Passenger Backrest for Rebel500 Rebel250 Rebel 250 Rebel 300 CMX500 CMX300 2017-2021 Pad Sissy Bar https://www.amazon.com/dp/B09DVPTSMN/ref=cm_sw_r_cp_api_i_465VY7K4SDXCRG88A7T2?_encoding=UTF8&psc=1
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“Viking Battle Axe Honda Rebel 500 Motorcycle Sissy Bar With Backrest – Vikingbags” https://www.vikingbags.com/honda-rebel-500-battle-axe-motorcycle-sissy-bar-with-backrest?gclid=CjwKCAjw0a-SBhBkEiwApljU0qc6GEwNZAm0JZLA3INjAwsL-CdsApjB9-09NCt8iFQJvinKXwNrhhoCEV0QAvD_BwE
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T-Rex: https://www.t-rex-racing.com/Honda-Rebel-300-500-Back-Rest-Grab-bar-Rack-p/n27-20br.htm 
Rebel500 