Where can I find the plastic license plate clips/nuts that the screws go into to hold the license plate?
The universal square ones (pictured below) do not fit in the license plate holder on my '01 VDP and can't be modified to fit

 

They do, but it is very tight. The pieces are also very stiff.

Squeeze the flats at the base using a very fine pair of needle nose pliers, then push in from the front.

They'll fly across the garage a few times until you get the hang of it.

 

I tried and they do,eventually,go in but the license plate holder is too thick for the clips to click in.

 

You could perhaps carve down the ridge around the ovals.

Or, lube the holes and push a bit harder while the holder is off the car.

 

I'm an new member here so I can't see your picture but I purchased the white plastic nuts at my local Ace hardware store. They had a couple boxes of License plate stuff in there nuts and bolts section. They fit nicely on the rear bumper and the License plate holder attached to the front bumper of my car. I'm not sure it the front license plate holder on my front bumper is standard issue for the Jaguar so yours might be different up front.