Here's a site that has royalty-free, vintage graphics, including old postcards, clock and watch faces, advertisements and product labels, ledgers, receipts, price tags, shipping labels, etc. Many of these can be useful for modelers:
http://knickoftimeinteriors.blogspot.com/p/antique-graphics-to-print.html (http://knickoftimeinteriors.blogspot.com/p/antique-graphics-to-print.html)
Great site.
I do a lot of searching on EBay for COLOR LETTERHEAD and find all kinds of neat stuff.