I priced them at a sign shop and they seem to be high...$60 for 2 magnetic #'s or $25 for 2 vinyl #'s. I checked Lowes, but didn't see anything.
you need to get really good magnets for them to stay on at high speeds and you'll still probably need to reinforce them with tape. I've found that out the hard way.
I'd suggest SPS/Solotime.... they sponsor National Solo and many other grassroots motorsports events..... Here is their line of track day re-usable vinyl numbers.... http://www.soloperformance.com/Track-Day-Numbers_c_50.html