Dell and Sony do have part numbers for everything. It is just whether you can get them to sell you the part that doesn't also involve them sending out an engineer to fit it for you. Apparently it is easier to get spare parts out of Dell if you're in the US since there is some telephone number you can call that will sell you the spare. If you're in Europe then it'll be quicker to trawl eBay or find a local computer recycler.