I got the 5.0 AH battery from zbattery.com. I'm sure you can get one locally for less than $15. The electronics store a few blocks away has them.
Seems like if you reduce the amount of power you will dim the bulb. I would go with an LED taillight to reduce power consumption. The 8-LED I got draws only 0.05 amps, that's 5 one-hundredths of an amp. It was $6-7 plus shipping from led-r-us.com. They have others, though not with the reflective lens, up to 61 LEDs.
Yesterday I put on my prototype bracket and mounted my light and a toggle switch. I even found a boot to keep water out of the switch. The boot replaces the switch's mounting nut and forms a weather tight seal. I used some of the sealed wire nuts for the exposed connection for right now. It was raining on the way home, and everything worked fine. If I have a chance today I take a close up picture of the bracket. It doesn't look bad at all. Still have the battery in my bar bag for now. Next phase is to create a bracket for mounting one or two of the LED taillights and strobe.
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If it ain't broke, mess with it anyway!