Dell does not sell the carriers directly (without a drive), but the carriers are labelled with a part number, which is sold by many suppliers. X968D does appear to be a 3.5" carrier compatible with the R510.
There are different holes to align to SAS or SATA drives.
Not all carriers fit all servers. CC852 is for the older 9G servers.
The carrier labels/stickers are a different shape on the new carriers versus the old carriers. WJ398 is the older (6-9G) label (not that you couldn't use them anyway, they just wouldn't fit in the "spot" for the labels). I could not find the "new" label sheets either.