I’d be surprised if there’s such a thing as a kit, as all you need is a replacement seal.
So go for the get the genuine item, unless you want to do the job again in a few years, STC3124 was the pt no. Although it may have been superseded now.
So for genuine parts why not try Rimmer bros. https://rimmerbros.com/images/PDFs/RangeRover_P38.pdf