You can't buy the Mystic paint from the Mustang. I believe it was BASF that made it, and to get some, you need to have a representative from BASF come to the dealer to inspect the damage to your Mystic Mustang and they will ship what they feel is an appropriate quantity to do repairs to the dealer.
Having said that, just about every paint manufacturer makes color changing paints now. I've heard that PPG's line is supposed to be good.
It is expensive, but I don't think it's $2500 a gallon. I think it might be about $150-250 a quart. I was going to have my bike done in it, and it was going to cost something like $800 for the materials, and I think that was 2 quarts of color and 2 quarts of clear.
I saw a Viper at VOI5 that was done in the color changing paint, and it looked pretty nice. Also, Bob Woodhouse's race car is done in some sort of color changing paint. He told me how they did it a while back, but I forgot. I think they mixed the color changing paint with something else to 'dilute' it. Bob's car has a pretty subtle effect - if you use 100% changing paint, the effect is dramatic. Bob's car just sort of shifts hues from blueish to a purplish tint or a greenish tint in certain light - looks nice!