Try
http://cadillacsonly.com/
They are a big salvage yard. So, the gasket will not be new and it might be a bit pricey but sometimes they're the only game in town. Maybe someone else will chime in with a better idea.
I wish that I woudl have forseen the fact that car manufacturers only make spare parts available for limited number of years. I could have stocked up with new parts when they were still available in the early 90s. At that time I was driving 84 Eldo. But I didn't know then that I would own 85 Eldo in year 2007... Such is life...
Peteski