There are sign blanks and then there are sign blanks. Sometimes they're even called sign blanks. They might also be called borders, frames, panels and who knows what.
Signs blanks, as a term, is used to describe all of the above. Mostly they come as vectors ready for vinyl cutting or for adding your own fills. Sometimes they come as bitmapped images all setup with multiple layers ready for print and cut.
There are 1,000's of vector images at
ExpressClipart.com that can be described as sign blanks. They are also sprinkled throughout the
Vector Art Mega Collections in vector form and there is a large group of them in the
Vector Art Digital Mega Collection in print and cut form.
Aurora Graphics also has a nicely done series of them available on their own or as part of the Print Craft Suite.