Haven't seen any besides what I've posted in terms of creating your own from scratch or emailing a cool e-card with museum art?
Want to receive or give a gift of creativity?
Kat Eye Studio hosts post card swaps--visit her site and join the "Liberate Your Art Post Card Swap." According to Kat, this swap "encourages you to create, share, and liberate your art out into the world, where it can do its greatest good." You'll need to sign up soon to be part of the fun!
Also, check out postcrossings and The Post Card Press for more ideas.
The bottom line--bring delight to the mailboxes of the world and YOU!