There have been several threads discussing battery life and chargers and the consensus is that a 1.0A charger should give you a little more juice that the .7A charger that comes with the Epic without damaging your phone. I have seen a couple people ask if a 2.0A charger would be too much but I have yet to see anyone come back with a definitive yes or no. Since the phone charges off of a micro-USB cable it should be pretty simple to find a car charger with a USB port that has the amperage that you want.
I found this in about 10 seconds on Google:
Newegg.com - Rosewill RCP-32TU USB Car Charger with 2 USB Port at 1 Amp