libva-nvidia-driver was recently imported to the Arch extra repo and removed from the AUR.
See: [PRQ#50485] Deletion Request for libva-nvidia-driver
If you don’t want to wait for it to reach the stable branch, you can build it yourself manually just like you would an AUR package:
sudo pacman -Syu base-devel git
git clone https://gitlab.archlinux.org/archlinux/packaging/packages/libva-nvidia-driver.git
cd libva-nvidia-driver
makepkg -srif
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