I would think you should be able to use any 12v battery. Personally, I use a 12V 7ah battery such as this one:
http://cgi.ebay.com/12V-7AH-SEALED-LEAD-ACID-BATTERY-/180597320806?pt=LH_DefaultDomain_0&hash=item2a0c706c66
Also, sometime ago, Lowrance used to see similar batteries to use with their Airmap 1000 and 2000 systems. I've found that these batteries will power any handheld well over 6 hours without any problems. You can find 110V chargers for these batteries at a low cost at about any Walmart or Radio Shack.