Heres a web link Ive used here in the UK for reproducing old signs (they even come up to year 2000) whilst its a UK site it does have a wealth of US ads and registration is free and some clever spark has set it up to accept both UK and US address formats. The ads have a faint watermark in them but when reproduced at a suitable scale its invisible
http://www.advertisingarchives.co.uk/index.php
Shaun