noahwitchell.com
http://www.noahwitchell.com/freebies

locationtextures.com
https://locationtextures.com/panoramas/

maxroz.com
https://www.maxroz.com/hdri/list

HDRI Haven
https://hdrihaven.com/

Poly Haven
https://polyhaven.com/hdris

Domeble
https://www.domeble.com/

IHDRI
https://www.ihdri.com/

HDRMaps
https://hdrmaps.com/

NoEmotionHdrs.net
http://noemotionhdrs.net/hdrday.html

OpenFootage.net
https://www.openfootage.net/hdri-panorama/

HDRI-hub
https://www.hdri-hub.com/hdrishop/hdri

.zwischendrin
https://www.zwischendrin.com/en/browse/hdri

Longer list here:

https://cgtricks.com/list-sites-free-hdri/