These spheres are used for look-dev and lighting references in VFX, they are not used to photograph HDRIs or backplates.
It's been a while since I wanted to own and to be able to use a grey ball and a chrome ball on set for reference. I was looking online to purchase a kit but they are very expensive for what it actually is. Besides, by creating it myself I could actually think about the design to make it very versatile. I can actually mount the kit on any standard light stand that are usually use on set so I don't have to travel with an extra camera tripod to set then on to.