I'm a little confused as to the location of the wedding. If it is in St. Louis, here are a couple of links that might be useful, discovered during a very quick google search.
Champagne Limousines in St. Louis has a white Rolls Royce Silver Cloud among its fleet of limos. It only has room for two, so it would only fit the bride and groom. Also, it isn't black.
St. Louis appears to have a Bentley dealer, so maybe they could give your mother some suggestions as to how to locate a black model.
I also found a couple of places that have classic cars; this first one actually rents them, but they are more from the 50's, 60's and 70's.
Classic Cars Plus: http://www.classiccarsforrent.com/
This place sells classic cars, but they might steer your mom toward some place that rents them. Hyman Limited Classic Cars: http://www.hymanltd.com/
To answer your question as to what is a Packard, it was a luxury American car manufacturer that went out of business back in the late 50's. They are very beautiful cars, and highly prized by collectors.
Finally, I doubt very much that the Classic Limo place here in Minneapolis would rent your mother one of their vehicles, at least not for anything less than thousands of dollars, but in a city the size of Saint Louis there must be some place that provides such a service. I hope I've given you some good ideas to start the search. Good luck.