I had a 3rd party one crap out on me after my original wore out/died. The replacement part from Dell was much better than the one that came with it. Just go to Dell.com and find the accessories section.
The problem with those cords is the batteries in them are connected in series, so if one cell goes dead the whole thing is useless. So if you know how to test the cells, you can take apart the thing & replace the bad cell. But that is a large pain in the ass compared to just buying a new one..