I was doing the mad scientist thing at home depot trying to revamp my light.... bought some NiMH batteries on line so I was looking for connectors... at any rate I stumbled across some 6V sealed lead acid batteries for emergency lighting. They were only $12.95 for the 6 amp hour battery.. that is an AWESOME price. If if hadn't already bought the NiMH I would have used these instead. On edit, they were the perfect size for a bike headlamp setup WITHOUT using a water bottle cage. They were smaller or about the size of two decks of cards put together, maybe 5x6 inches and fairly lightweight.
Yeah the Nimh batts are better for certain BUT.. if someone out there is trying to put together an inexpensive system from scratch you simply could not beat this price. I got 4, 4 AH 6 Volt batteries online for 56.oo shipped. A great deal, BUT if I was doing the home depot deal for a headlight I'd go wigh the SLA 6 AH for 12 bucks.
I use a lead acid battery stuffed into a waterbottle to power my dynamo-style headlamp. Its a cheap and effective system, that is easy to maintain. The charger switches to trickle-mode automatically, so is better than most shop-supplied budget models.