I've been looking for one too because ours went flat during out extended stay at home. Take a look at the Duralast Platinum Battery 35-AGM Group Size 35. According to Duralast it fits the RX 450h.
https://www.autozone.com/batteries-starting-and-charging/battery?filters=4294914202&aqs=4294914202&cmpid=DUR:US:EN:AD:NL:1000000:DBA:DLPlat-Battery