you'd have to buy it either as a spare part from Dell or find it on Ebay or Amazon or somewhere. Installing it is pretty easy, its a plug-in Mini-PCI card.
Need to download the software drivers as well.
I'm still looking for a combo Mini-PCI card with both WiFi and GOBI 2000 for worldwide WWAN connectivity..
Ive contacted Dell to know if they can sell and ship it for me. On eBay and Amazon, the price is about 60$ ! and you can have it with a new PC on dell webstore for 15/20bucks ... so, hope that Dell will be able to sell it separately...