Look for stuff like this

https://www.amazon.com/Lixada-Pannier-Bicycle-Cycling-Grocery/dp/B07V9MTYR4/ref=mp_s_a_1_85_sspa?crid=1JUQF1IK0QGM4&keywords=bicycle+side+bags&qid=1651580144&sprefix=bicycle+side%2Caps%2C423&sr=8-85-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExVTM0WFAxVU1WVUFJJmVuY3J5cHRlZElkPUEwNzk0NjA4M05KRUZFNExDT0ZXSCZlbmNyeXB0ZWRBZElkPUEwMDc1MzUwM04xMUhZSFNKWTBBRyZ3aWRnZXROYW1lPXNwX3Bob25lX3NlYXJjaF9idGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Make sure that they're "stiff" so that they're not flapping about by your rear wheel