A number of jewelry supply companies sell these great adhesive earring card adapters. You can design your own hanging earring cards for display at craft shows.

These are the ones I made. I used Zazzle to design and print the cards but you could get more handmade with art paper and pens or rubber stamps. When I designed the cards I added dots so I'd know where to punch the holes and they would all be uniform. To punch the holes I just used a safety pin and one of those cheap foam sponges from the dollar store underneath the card (a towel or pillow works as well).

I bought my earring card adapters from a seller on Etsy who was clearing out extras. A few stores that sell them are:

If you would like to have your cards printed on business cards like mine here's a template you can use that is at 300 dpi which is what most printers prefer, just click to view and save the full size version. You'll want to cover up the gray border and text but keep the hole markers for printing.