Radio Shack has 8xAA battery packs in a couple different configs and the connectors. All are cheap (less than $5).
Use alkaline batteries for goofing around and testing etc. but on a trip Lithiums really are much better saving weight and increasing run time. DON'T forget to turn off the unit when you are done using it! One set of lithium batteries can get you through a trip of nine days using the unit judiciously (just when fishing). Two sets should allow using the locater whenever you're on the water.