Thing that holds my battery into my camera is lost- what do i do?

I have a nikon D3000, the little thing that holds the battery in fell off and is lost. I've been using cardboard and tape, but thats not working out great. I don't know what that piece is called- where can I get a new one? Or is there any better ways to make that work?

Do you mean the battery door?
Those you can buy and easily replace.

Check the Nikon website to see if they have the part number or just search for a D3000 battery door.
eBay will most definitely sell them.