For the tines I used these 4oz clear top tins from Specialty Bottle. After using them for nearly a year I can tell you there aren't quite airtight and your spices will age if not used quickly. They also sell 3oz solid top twist tins, which I believe are more airtight, although also more expensive and you lose some of the aesthetics. As for the magnets, I found these 1/2x1/16" self-adhesive neodymium magnets are great. No messing with multiple, tiny magnets or mixing your own epoxy. The magnet is large enough to easily sit flat on a counter, but thin enough so if it does tip to one side nothing will spill out.
I had my design printed on vinyl at a local shop, and they applied it to some 403 magnetic stainless steel sheet metal I bought from StainlessSupply.com. I got the labels from LabelsByTheSheet.com and printed them myself. I made the frame with some moulding and spray paint from Home Depot, but you could also buy a frame from a craft store.
EDIT: WOW! I never expected to be featured on the Instructables homepage, let alone ThinkGeek's and "IFLS"'s Facebook page. Thanks to everyone who "liked" this project. Unfortunately, there is a patent on "magnetic spice racks" and I will not be able to sell this or start a Kickstarter because of the overwhelming patent licensing/legal fees. However, I will sell PDFs of the background through the link below. If you'd like a custom version of the background, let me know and I can make you one for a few dollars extra. The font I used for the labels is just Arial and the words are Garamond. There's obviously nothing to stop you from making your own version from scratch, but please honor my copyright of this specific design.
Thanks and enjoy!
Purchase the PDF. No Instructables Pro account required. Note that in the plain white picture above, I've changes some of the spices in the blends area and the version for sale has these changes.