There is blank DVD-9 media for sale, and it is called double layer for +R and dual layer for -R, though I tend to just refer to all of it as dual layer out of bad habit. DL is a good abbreviation. You need a burner capable of burning this media, but all modern burners made in the last four years or so can do this. If you want to check the capabilities of your particular burner, download and run Nero InfoTool and it will list the various types of media it can handle.
Nero InfoTool is free to use. http://www.nero.com/enu/support-nero8-tools-utilities.html
The best, most consistent DL media is sold under the Verbatim brand, and you should stick with the +R DL for best compatibility. This point needs to be emphasized....many of the problems we see with burning DL media is due to poor quality disks. So stick with Verbatim.
One of the other problems when burning video to DL is getting the layer break set correctly. I recommend burning with ImgBurn, since it has an excellent record with this task. ImgBurn is also free to download and use.