Recently, one of the brackets for my passenger side running board snapped. I cannot find a single one for sale online, even as a pair. Is anyone looking to sell or know of a place that is selling?

If it's just a running board like you say, just buy the whole unit. But if it's deployable sidesteps that you have fitted, then I'm afraid you are right out of luck as they are like hens teeth and should you source a bracket it's going to be very expensive.

Stuart on here has reproduced deployable step brackets. They are around £90 each1 from memory.

Stuart on here has reproduced deployable step brackets. They are around £90 each1 from memory. His name on here is Oggy.

My running board brackets were in a terrible state when I bought the RR, near rusted through, I made 4 new ones from a length of 4mm u-profile painted it with Hammerite and every other black paint I could find and they are fine 4 years later. If you don't have the facilities to do it yourself get a local metal basher to fabricate them.
If you do go the new parts route give the brackets a generous coat of hard wearing paint, they do get a hammering from gravel and other road debris.