I've used Airnav.com. Give it a try at:
http://www.airnav.com/fuel/
You can check prices near an airport or planned destination (just as you described), or can plan a X-C with fuel stops. It also has great airport information under the airport tab.
Happy Hunting!
Cheers,
Bob Mills
"Rocket" RV-6
N600SS
4SD ($4.65/gal for 100LL)