To find such a thing, I was forced to make (read: get dad to help me make) my own. So here's what we did:
After measuring the cabinet, we went to Lowe's and found a piece of steel that was the right width but 1" too long. It's possible that the store may have cut the metal for us, but we took it to my dad's house and cut it with a hand saw instead. My dad then used his grinder (wire brush attachment, I think) to smooth all the edges smooth* and to sand out the rust spots. We then wiped it down with turpentine to clean the surface (don't use water as this will generate rust). I then spray painted (see, I did some of the work!) both sides with two coats of silver paint. Lastly, we used some epoxy glue to glue the metal to the cabinet door (we had removed the cabinet door and brought it with us to my dad's house). I piled some heavy books on top of the metal and let it sit for 24 hrs. Finally, I re-hung the cabinet door and, voila! Customized magnetic spice rack.
I used post-it notes stuck to the back of the spice jars to label them b/c I didn't want the labels to show. Some may argue that you don't need to label your spices as you can see inside the containers, but (1) some of my spices look alike, such as my Pampered Chef Moroccan Spice Rub and Cajun Spice Rub, and (2) even if I know what they all are, my husband may not, so I feel it's best to label. You could also use a label maker and affix the labels to the side of the container, but as I said, I didn't want my labels to show. Plus, post-it notes are a less permanent solution, in case you change the contents of your spice containers.
I've also seen these types of containers used as office supplies such as paper clips, thumb tacks, etc. Perhaps my next project will be to buy some more containers and use them to organize my office supplies!
*The metal has very rough edges, even the uncut sides, so if you attempt this project, handle the metal while wearing protective gloves and do not skip the sanding step! You don't want rough edges in your kitchen.