For the labels I used these:
http://www.amazon.com/Avery-Weatherproof-Shipping-Labels-8-5-Inches/dp/B00006IBUY/ref=sr_1_5?ie=UTF8&qid=1422454176&sr=8-5&keywords=avery+weatherproof+labels
I bought a 10 pack from Office Max. I could not find single sheet 8.5x11 Avery weatherproof vinyl laser labels, but having two 8.5x5.5 labels on one sheet doesn't pose much of a problem. They butt right against each other and when you apply them to the car you can barely tell. (There a few other brands on Amazon with full sheet 8.5x11 laser weatherproof vinyl labels but they don't have free shipping and come in packs of 100 so I didn't use them) I tape the seam with blue painters tape and apply the label like a Band-Aid, where I start on end, and then slide the backing out, which pulls down the remainder of the label. I have a helper to hold the magnet and help keep the vinyl laying down flat as I apply it to reduce air bubbles.
The magnet sheet I bought was this:
http://www.amazon.com/gp/product/B000PSL68C/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
It is 1/64" and is very close to the thickness of the round pre-made magnet that I bought. It was very easy to cut with scissors too.
I also bought a sheet of this because I read that .030 mil is the thickness to use, but it was much thicker than my premade commercial round magnets and so I used the thinner sheet, which seems to stick to the car very well.
Flexible Magnetic Sheet, White Vinyl, Sign Blank, 0.030" Thick, 12" Wide, 24" Length (Pack of 1): Industrial Flexible Magnets: Amazon.com: Industrial & Scientific
Both sheets came in a roll about the size of a paper towel tube, but I unrolled them onto the side of a metal filing cabinet and they flattened out in no time.
Another possible option instead of weatherproof vinyl shipping labels would be something called "laser vinyl" which comes in rolls, and is similar to what you get when you order an adhesive vinyl decal except that you can print on it with a laser printer. I couldn't find any of that cheap enough, and I believe that the laser vinyl shipping labels are very similar and more convenient in that they come on sheets.
A couple of folks asked why I didn't just order new ones, or what was wrong with my premade magnets. I ordered them from TrackDecals.com which I love and is where I order all my track outlines from. For Christmas I ordered a total of 8 magnets: Car numbers for me and my son, and SCCA class magnets for me and my son. I specified 8" and 4" and round magnets. What I got was an 8" magnet with car numbers only about 3" high, with no attempt to lay them out and fill the circle. The 4" class magnets had letters that were around 1.5" high. It was mostly white space above and below the numbers/letters. Totally tiny and unusable. They came in right before an autocross and they let us run with them "this" time but fix it before we come back (I wish I had a picture) Other than that they looked great. I didn't want to spend another $75 on more magnets so I decided to make some and get exactly the fonts/design I wanted