Does anyone know a place that has them?

 

It's a 2000 Disco II, 4.0 V8.

 

If you are talking about the cap with the fluid height sensor in it, you will probably need to find one from the dealer.
If it has been cracked for awhile and your fluid hasen't been changed for awhile, I would suggest you do a fluid flush at the same time. Your old fluid is probably burned and full of moisture due to the exposer to air from the cracked cap.
Mike