Are the keys cracked or anything? Look at the bottom of the keys- if nothing looks cracked, they should pop back on. It takes a bit of effort, but they should slide back on.
Dell does not sell keys, I'm sure its too late for you, but might help others. I bought some laptop keys for my Dell Studio 15 at individualkeys.com, they are great, fast and cheap.
Hey I had the same problem a couple of months back and I was about to order a new replacement keyboard but one of my friend suggested me for "www.laptopkey.com" and I ordered my replacement keys from them and saved a whole lot of money. They have a great customer service. I will strongly recommend this to you. They also have understandable video installation guides to help us fixing our keys. You can check them out.