You could use one from a sprue of the Fireforge Teutonic mounted Knights and a dismounted one from a sprue of their footknights as they have the same helmet options iirc.
Curteys/first Corps do mounted and unmounted knight packs, which might be the cheapest option but might require conversion or just a suitable paint job.
The other option would be to scour eBay or put a wanted post on this forum for an old mounted and dismounted warhammer fantasy chaos knight?