I've been looking for a card to hand to restaurants and found some on Humane Society's page. If designing the card from scratch, I would take out the pledge part (more suitable for Western-style communication) and use one of Rob Stewart's gorgeous shark photos.
For weddings I found this card to explain to the older generation, via browsing a Singapore wedding site where someone posted: i just went to a buffet style wedding and half the people there boycotted the shark fin soup (myself included) so it was even more wasted loh. This postcard is from WildAid. If you need a stack of them email WildAid (Victor Wu, email@example.com, or whoever is handling it at this time).
If parents object, maybe explain that sharks have the most mercury, since they're top of the food chain and toxins accumulate in their system as they eat other animals. Mercury (from our pollution of oceans) is a neural toxin that damages the brain and nervous system and can cause sterility in men. People who eat seafood regularly already consume too much mercury <www.chinadaily.com.cn/english/doc/2005-05/21/content_444520.htm>. People who're for not eating shark-fin include Yao Ming, Lisa Ling, Ang Lee, Jackie Chan (video PSA's at WildAid), and other people endorsing WildAid are Zhang ZiYi, Jackie Chan, Maggie Cheung, Chow Yuen Fatt, Michelle Yeoh, Joan Chen, Tony Leung, and more. Soup alternatives at these wedding sites in Singapore and Malaysia.
This couple who made their own elegant card for their wedding also has a page to explain it. And yes, you can order their cards or download them.
Each year almost 100 million sharks are killed to make shark's fin soup. Many times a shark is dragged onto a boat and its fins cut off, then tossed back into the ocean while it dies a slow and painful death. This practice is not only cruel but wasteful. (www.sharkwater.com)
Many species of sharks that are caught are friendly. Deborah Peterson wrote, "In testament to how friendly some sharks may be to human advance, yours truly actually braved the shallow waters to hug a shark once ... What I found most surprising was the texture of the shark's skin which was rough to the touch, with a very firm body -- not fat, blubbery or slippery as I had imagined. The shark seemed to take my advance to pick it up without a hint of struggle or fear of humankind, in a sense, showing a trust" (photo of Deborah hugging shark at www.lifeinthefastlane.ca). Photos of a hammerhead shark that's finned below (from www.protect-the-sharks.org).
Many sharks are facing extinction including the hammerhead shark pictured above. Below is a photo of Rob Stewart swimming with reef sharks (www.sharkwater.com).
Here're some cartoons by Mutts: