Check out the "parts for sale" section on www.crownvic.net. P71 PCMs often come up for sale. In addition, you could post what you want in the "parts wanted" section.
I am not sure if you would have to use a P71 computer from a '93 or not. However, I am almost positive the '98 and up units would be incompatible.
You can also have a custom chip burned for you that will give you the benefits of a P71 PCM with other benefits such as firmer transmission shifts and an eliminated speed limiter.
Here is one such company:
http://www.reinhartautomotive.com/index2.ivnu
Here is a link to help you with the installation:
http://www.crownvic.net/liquid/chip.html