You'll need to check the stud pattern / spacing and offset - this will be the guide as to what you can fit.
I'm not familiar with the crown, but I would have thought it would probably have a relatively normal setup, so you should be able to find compatible rims. Your best bet would be a tyre shop - they should know what will fit and what won't.
Just remember that there are different wheels out there - get the wrong ones and you'll either not fit at all, rub on something, or do odd things (including damaging) your suspension. I guess I'm trying to say that you need to check and be confident that you are getting the right wheels for the car.
I just had a quick look at http://www.azcwheels.com (LA based), and it looked like they had 36 different rims to fit a 2000 model crown vic . . . There will be many sites like this out there . . .