Are you talking about the signs that you "display" when you are at a car show?
Any competent graphics business should be able to get you what you need. You just need to decide what type of material you want the sign made out of...plexiglass (colored or not), metal, coroplast, etc...
For that matter, you can design your own sign on your computer and take it to a Kinkos and they can print it for you.
EDIT: I found these guys on the net...though you can still do it yourself!
http://www.carzigns.com/