Parts-People is the most reliable source of genuine Dell parts. They have parts for your laptops.  Batteries from eBay, Amazon, etc. are a gamble. They can be old Dell batteries which would be degraded and not perform well, or a Dell batteries that were "refurbished" by someone else (not Dell or approved supplier) by replacing the internal cells, or outright counterfeits. Such batteries might work, or might not, or might fail before long. They could also damage your laptop. If you buy such a battery, make sure to choose a vendor that has a good return/refund policy. As for cables, asking Parts-People would be a good place to start. They respond to email inquiries. BTW, what country are you in?
I've been thinking of refurbishing some Latitude 7480 and 7490 laptops to sell. They are missing some parts or some parts need to be replaced. The trouble is that new batteries directly from Dell are about $100 per battery. The laptops sold as used aren't worth much more than that on eBay. Is there any hope of getting a battery from eBay, maybe even a used battery, that isn't a knockoff? What about the internal battery cables? I ordered a couple of them from eBay just to check them out and found they are slightly longer than they are supposed to be. They will fit in there with a bit of bending but seems like ideally that shouldn't be needed. Might the battery cables be counterfeit too? Just wondering if any one has had experience refurbishing these laptops. Any suggestions for what to do about getting parts?
Latitude 7490 batteries and internal battery cables