I have the 65 GT fog lights. The bulb I belive is the same. I have asked several Mustang shops etc and the answer is no there is not.
I actually think they work about as good as most newer car's fog lights do.
If you want more light try some Sylvania Xtravision headlights, and a relay setup. It is a very simple modification.
The regular halogen bulbs are something like 35 watts low and 40 high. The Sylvania XV bulbs are 55-65 like the late model capsule bulbs.
The best part is that these bulbs look stock.
The factory switch and wiring was not designed to run halogen bulbs to begin with, and definitely not higher wattage ones.
You can go this route.
http://www.midnightdsigns.com/james/headlights.htm
I have this.
http://rjminjectiontech.com/?p=8
You should also try aiming all the lights. That is a significant factor.