ebay,craigslist, garage sale,home depot,lowes,ace,grainger, secondhand shop, sears, or the actual companies website like craftman,black and decker,ryobi ect.
+1.Home depot, Lowes, Sears,and single purpose battery shops may have your model also. (single purpose battery shops can be found in your local yellow pages or similar. We have at least one in Cleveland, and I'm sure they exist elsewhere
You could try direct from the manufacturer(may be a bit expensive), or try ebay(may be a bit cheaper). I hate to recommend google, but type in the battery model number followed by replacement and see what comes up. Worth a shot.
Ahh well that depends if it is a craftsman try your local Sears store. If its any other, assuming you live in the states, try Batteries Plus. If they don't have it in stock they will order them for you.
on sears just type in the search the batteries voltage eg. "15.5v Batteries" www.sears.com/
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on sears just type in the search the batteries voltage eg. "15.5v Batteries"
www.sears.com/
www.batteriesplus.com/manufacturer/50-0/Cordless-Tool-Batteries.aspx