The Frugal Crafter has this sheet of valentine kewpies for downloading here. She made lovely little lollipop covers with them.
Altered Stuff has a wonderful collage sheet of vintage valentines that can be downloaded in pdf or jpg format.
If you want to send vintage ecards, West Virginia University Library has put together a selection of valentine cards and postcards. This is a sample from their collection:
The valentines that I used fort my ATC's were from Zwani graphics. This site has html code ready to cut and paste so that you can add a wonderful vintage valentine to your site!
My ATC looked like this