There are a lot of aftermarket batterys that will work, you can buy them almost anywhere but in my opinion the harley battery is the best and is sold at their dealerships.
Which ever battery MFG you choose, I would recommend an "AGM" type technology... It's spill proof & seems to last much longer. "AGM"= Absorbed Glass Mat.
There was a post a couple of weeks ago from a guy who was fighting an electrical problem and he had two batteries load tested, a new aftermarket battery and an older
HD battery. He said the difference was incredible, the old HD was still much better in
terms of CCA. I wouldn't put anything but an HD in mine, always have. Yeah, they cost more, but are worth it IMO.
I have a 2004 Sporty and there are NOT nearly as many aftermarket alternatives other than the OEM HD due to it's special size and tight "nesting" place...been there! The good news is the HD battery is very very good and I would recommend it highly, just remember to use a Deltran or HD Battery Tender if not using the bike for more than a month or so to keep the charge up. Fully discharging a modern high density, low electrolyte/water level battery will certainly damage it's capacity and life.