This one was filmed in the NY area, so I would check the film car sites that service NYC first. The problem is that this movie is several years old now. The only ones I found were close, but no exact match. It's also possible with a specialized vehicle like this, that they rented one directly from a local funeral home.

http://imcdb.org/vehicle_543409-Cadillac-DeVille-Flower-Car-2000.html

AnimatronixX was able to identify the exact model and coach builder.