Different proxies can be recommended.
https://www.manticgames.com/games/kings-of-war/undead/undead-revenant-cavalry-regiment/
You could try these. Mantic was set up as a producer of not-Warhammer miniatures. These are your Black Knights.
Why do you want to do this, is it because you don't like Black Knights or can't find Black Knights.
If the latter.
The current range of Vampire Count miniatures which include Black Knights are called Soulblight Gravelords.
GW does not allow direct retail of Black Knights from discounters, they have the 'evil rune' arrow, but the Start Collecting: Soulblight Gravelords set is available without restriction.
This contains 5 Black Knights, 20 plastic Grave Guard, and a mounted Wight Lord character. You will have to source square bases separately.
I recommend the plastic Grave Guard, the set is frankly very good and the discounting on Start Collecting compounds well with an online retailer discount. Two of these sets will provide a decent core for a wight heavy Vampire Counts list.
If you only want the Black Knights then because it is a component of a discount Start Collecting set a number of sellers will sell you the unit separately. You will not have to wait long for a unit of five to turn up on ebay in your delivery radius.
You can also buy Black Knights via older Start Collecting Skeleton Horde, or Start Collecting Malignants. The latter set, Malignants may appear not to include Black Knights, but it does, they are the same kit as the Hexwraith models showcased which can be assembled as either model.