I found these postcards you can request a pack of.
From the request form:
SEND a personal letter asking your country's leader to support effective, sustainable programs that save lives or SOW the seeds of change in your community.
By signing the petition, you have added your name to thousands of others who stand together to help reach the goals of THRIVE. Now go a step further.
Recruit and organize others: Fill out the form below to request cool new postcards made of plant-able carrot seed paper. Then use these postcards to host a letter writing party with your friends and family to write a message to the G8 asking them to help us achieve the goals of THRIVE when they meet in May. We'll also send you instructions on how to write your very own letter to the editor of your local newspaper.
Here's the clickable image to the order form: