Here's the scoop:
Anything and perhaps everything a bicyclist would want to know about lighting may be found at:
Bicycle Lighting Systems
There's a section about batteries, including a nice one on SLA's. Here's the link for that specific section.
Starlight 78 Bicycle Lighting System
In that section, you can click on the price button for a 5Ah SLA, and it will conveniently redirect you to zbattery.com, where you may order it, and trickle charger. I can't "vouch" for this company (zbattery.com), but the author of bicyclelighting.com recommends it.
One last thing, as I stated before, you will intantly be a "chick magnet" after installing your lighting system, and the autor of bicyclelighting.com has shared many of his encounters on his website. You may ignore the pictures with blacked-out censoring on various areas. So if you are squeamish about about nudity (religious, etc..), you may consider avoiding this site. However, the viewer also has the option to further investgate the photos, if desired, by double clicking on them.
It is imperative that you test the SLA battery (if purchased over the counter) before you buy it with a voltmeter. It should have a full charge (13volts for so or 12V, depending on your meter) on it, because SLA's stored for any length of time without a full charge will soon not be rechargable. 12 volt is nice, because cars and bycles run it, and lights/horns are easier to find. The author of bicyclelighting.com discusses the pros/con and things to watch for when buting batteries. I glad that he did, because I almost purchased a quite overpriced battery that was "dead from the start" at a local Radio Schack. If I would have purchased that setup, I would probably only have 20 minutes of lighting time instead of the 1.25 hours, and probably would not have known any better.
Personally, I paid full retail at a battery convenience store near my residence. (Batteries Plus) They offered a full one year guarantee with the stuff, but I had to pay about double price up front to avoid shipping delay, but did not have heavy shipping charge and got the 1 year guarantee.
Warning: Some minor nudity on site, so if squeamish or religious about it, you may consider avoiding site.
You may want to also consider Norm's lighting system. I haven't seen it, but hear that it is pretty good.