Is that a Halogen based system? If so, I just bought a water bottle battery and charger from Batterystation.com for my NiteRider dual handlebar mount for $85. It works great, though you will need to do a little soldering to get the whole thing together - its no big deal, but if you don't have access to a soldering iron it will be tough/impossible.
The NiteRider system I have can easily accept MR-16 bulbs, which is great since those bulbs are used in landscaping and can be bought at Home Depot ($6). I bought a higher voltage battery (14.4V), overvolting the bulb means you get much brighter/whiter light, though bulb life suffers. I figure I can get a whole season on a single bulb, so at $6 I'm fine with that.
If you have further questions, I can possibly help you out with the electrical stuff - its pretty simple stuff.