on amazon, this was listed , also saw OTF mentions on the comments section so seems like it fits

Scosche Rhythm+ Replacement Strap - Fits Scosche Rhythm+ Optical Heart Rate Monitor Armband

https://www.amazon.com/Scosche-Rhythm-Replacement-Strap-Optical/dp/B07K1MW727/ref=sr_1_45?dchild=1&keywords=silicon%2Bband%2Bwith%2Bvelcro%2Bfor%2Bheart%2Bmonitor&qid=1620177095&sr=8-45&th=1