Posting here is a great start
You can also consider having your students (I'm not sure how old they are) write letters of request to the chamber of commerce in the capital of each state (you can find this information online). They are all great at sending postcards and other great resources.
When I did this a few years ago, I found a flight attendant who was willing to send my students a postcard from where she landed (sometimes they would come several at a time in an envelope). Do you have a parent/community member with this sort of connection?
Good luck! This is a fun project!