Find a place to get magnetic sheet. McMasterCarr is a good place as any.
Print numbers off your computer the size they are supposed to be. Use some temporary adhesive to stick the paper to the magnet, cut along the lines with a knife, pick off the paper, paint the magnetic number any color (contrasting of course) you want with some Krylon spray paint (Krylon works very well).
I don't have a current rule book and I know the SCCA (national) has new number requirements for 2004. Numbers at least 8" high with a stroke (thickness) of at least 1.25" are minimum. Class numbers have to be something like 4-5" high and 3/4" stroke.
Helvetica Bold or Black is a good font along with Impact or Helvetica Compressed.
Toy is spot on, McMaster-Carr is a great resource for magnetized rubber sheet if you want to go the DIY route, but I got a sweet set of magnetic numbers made up by RSXjedi (RSXJedi? Something like that, his name is Derrick) up here in the Chicago area. I got two color numbers light blue/silver that look great on my NBP, and you can spec the size & font to meet your needs. I'm sure he would give you a reasonable deal, just send him an email or PM and let him know Gaijin gave you his name.
I went to edecals.com and got a sticker for myself, and then I got a sheet of magnet...from a sign store... and plopped it on, in the past I made cut outs... but they sometimes get blown off depending on how big they are... so I figured a big square magnet wouldn't get blown off..